I am speaking next week at an event on Brexit organised by the Philippe Kirsch Institute in Toronto. In my presentation, which will be based on the lengthy analysis in this post, I will lay out the legal and political faultlines that have been exposed by the litigation over the triggering of Article 50.
Four criss-crossing faultlines are present in the Article 50 litigation. These faultlines can be perceived in constitutional law and the two decisions that have so far been released — Re McCord  NIQB 85 and R. (Miller) v Secretary of State for Exiting the European Union  EWHC (Admin.) 2768 — but in some respects they are mirror images of fissures in the political system more generally.
As is well known, Article 50 sets out the procedure by which a member state can leave the European Union; in particular, Article 50 provides for a two-year time period within which a departure can be negotiated and at the end of which the member state will leave the EU. Article 50 must first be ‘triggered’, however, in a manner consistent with domestic constitutional law. The basic question in the Article 50 litigation is “who decides?” Can the executive branch of the British government trigger Article 50 or must the triggering of Article 50 be authorized by Parliament (and, perhaps, the devolved administrations)? My goal here is to demonstrate that the four faultlines discussed below provide a useful framework for understanding how the Brexit litigation and Brexit more generally will be resolved.
Form and Substance
The first faultline is form and substance. In purely legal terms the issue is the effect of triggering Article 50 on EU law as implemented by Parliament. In more broadly political terms the issue is the extent to which triggering Article 50 will have an effect on the powers of the devolved administrations in Scotland and Northern Ireland.
A consideration of the terms of the European Communities Act 1972 is the best place to start. Section 2(1) provides:
All such rights, powers, liabilities, obligations and restrictions from time to time created or arising by or under the Treaties, and all such remedies and procedures from time to time provided for by or under the Treaties, as in accordance with the Treaties are without further enactment to be given legal effect or used in the United Kingdom shall be recognised and available in law, and be enforced, allowed and followed accordingly…
The result is that directly effective European Union measures (Commission decisions, regulations and directives, and Treaty provisions couched in sufficiently clear terms) have effect in British law automatically without further Parliamentary action. Section 2(2) makes provision for delegated legislation designed to give effect to EU law obligations.
One view is that the 1972 Act is simply the “conduit” through which EU law flows “from time to time”. Indeed, the 1972 Act is expressly premised on exercises of the prerogative that would cause Britain to become a party to “the Treaties”. Without the prerogative, s. 2(1) would simply make no sense. Such “rights” as exist under this regime are “rights from time to time created or arising by or under the Treaties'”. As Professor Finnis puts it:
Treaty-based rights are statutory in that they depend for their effect in UK law on Parliamentary enactment; but they are not statutory inasmuch as they are not themselves enacted by Parliament and can be terminated (“destroyed”) by termination of treaties in the course of the Crown’s dealings with foreign entities or states.
Triggering Article 50 will simply “commence the formal legal process by which the UK leaves the EU, no more and no less“. Moreover, even at the end of the Article 50 period, the 1972 Act would remain on the books, ready to spring back to life if Britain were to re-enter the “Treaties”. But in the meantime, “there is simply nothing left for section 2 of the European Communities Act to bite upon“. It has even been suggested that the mooted “Great Repeal Bill” would not be necessary: ” repeal is not a necessary component of Brexit, if Brexit is understood to mean the extrication of the UK from its EU Treaty obligations”.
The opposing view is that triggering Article 50 will have the inevitable effect that, two years later, a series of rights in three categories — directly effective rights (such as worker protections) enjoyed in the UK, directly effective rights (such as freedom of movement) enjoyed by UK citizens elsewhere in Europe and derivative rights (such as the ability to stand for election to the European Parliament): “By issuing an Article 50 declaration, the Prime Minister would start the process that would inevitably end in the loss of EU rights (even if a way was found to negotiate a set of substitute, non-Treaty rights)“. In addition, triggering Article 50 would turn the 1972 Act “into what is in substance a dead letter” or even simply frustrate the purpose of a statute: “the key is whether the exercise of any remaining prerogative does, or does not, frustrate the intention of parliament in any relevant Act“. Moreover, it is difficult to see why the possibility that Parliament could in the future re-enact some (though clearly not all) of these rights would establish the lawfulness of issuing an Article 50 notice: the possibility that legislation could, in the future, ‘cure’ an ultra vires act is not one known to the common law.
In Re McCord, Maguire J. favoured form over substance:
On the day after the notice has been given, the law will in fact be the same as it was the day before it was given. The rights of individual citizens will not have changed – though it is, of course, true that in due course the body of EU law as it applies in the United Kingdom will, very likely, become the subject of change. But at the point when this occurs the process necessarily will be one controlled by parliamentary legislation, as this is the mechanism for changing the law in the United Kingdom (at para. 105).
As a result, triggering Article 50 would have no effect on the Northern Ireland Act 1998, two sections of which (ss. 6(2)(d) and 24(1)(b)) prohibit legislative and administrative action that would be incompatible with EU law.
Whereas in Miller, the Divisional Court favoured substance over form. Triggering Article 50 was said, in respect of the three categories of rights, to “deprive domestic law rights created by the ECA 1972 of effect” (at para. 64), effect a “material change” to domestic law (id.) and would “undo…rights which Parliament intended to bring into effect” (at para. 66).
An important political faultline crisscrosses this legal faultline. It concerns the effect of Brexit on the devolved administrations in Scotland, Northern Ireland and Wales. Although devolution operates quite differently in each jurisdiction some common issues arise. Do they have to consent (by means of legislative consent motions) to any legislation passed by Westminster to authorize the triggering of Article 50 or to sever all ties with the EU? In McCord, mindful that the Westminster Parliament retains plenary power “to make laws for Northern Ireland” (1998 Act, s. 5(6)), Maguire J. replied in the negative, on the basis that relations with the EU remain the sole and exclusive competence of the Westminster Parliament (see Schedule 2): “the better view is that any legislation for the purpose of notification under Article 50(2) would be legislation relating to an excepted matter i.e it would be legislation concerning relations with the European Communities and their institutions. It would not, in the court’s view, be legislation ‘with regards to devolved matters’, even if one was to adopt a broad approach to the meaning of this phrase”, as a matter of constitutional convention (at para. 121).
But this might be to elevate form over substance. For Maguire J. also said: ”The devolved institutions, to a greater or lesser extent, within the area transferred to them will be administering EU provisions and considering the future development of EU law in relevant subject areas” (at para. 106, emphasis added). It is unclear the extent to which areas such as agriculture, currently dominated by EU law, will be regulated post-Brexit by the devolved institutions. But Maguire J. can at least be read as suggesting that it will fall to the Northern Irish institutions to “administer EU provisions”, which raises the prospect that triggering Article 50 might indeed have an effect “with regards to devolved matters”, especially given that the nature of the regulatory regimes (now subject to EU law) would inevitably change post-Brexit. More generally, the devolved administrations are obliged to comply with EU law, an obligation that will be either strange or ineffective once Britain has left the EU.
The situation in Scotland is slightly different, but the same issues arise. Although s. 28(7) of the Scotland Act 1998 makes clear that devolution “does not affect the power of the Parliament of the United Kingdom to make laws for Scotland”, the relevant constitutional convention (the Sewel Convention) has now been placed on a statutory footing by the Scotland Act 2016 (s. 2, inserting a new s. 28(8) in the 1998 Act): “it is recognised that the Parliament of the United Kingdom will not normally legislate with regard to devolved matters without the consent of the Scottish Parliament”. As usual, the devil is in the detail: the word “recognised” suggests that the provision is simply declaratory of the existing constitutional convention, the word “normally” suggests that the convention does not apply in all cases and the words “with regard to” are ambiguous as to whether they only cover Westminster legislation on “devolved matters” or Westminster legislation that would alter the scope of the “devolved matters” on which the Scottish Parliament can legislate. These have been termed, respectively, the “policy” and “constitutional” arms of the Sewel Convention.
In the realm of constitutional convention, of course, obligations are politically enforceable, not legally enforceable. But depending on one’s view of the form and substance of Brexit, the obligations might be different. On the formal view, the devolved administrations have nothing much to fear from triggering Article 50: the existing machinery will continue to hum along, producing new outputs perhaps but operating in precisely the way it was designed to. And so the devolved administrations need not play a central role in the Article 50 process, still less a leading role. But on the substantive view, triggering Article 50 will start a process that will alter the scope of devolved competence fundamentally, such that the devolved administrations must be centrally involved. Indeed, it is not beyond the bounds of possibility that the UK Supreme Court might, after the fashion of the Supreme Court of Canada, recognize a constitutional convention in a way that makes compliance with it politically inevitable, or even impose duties, based on deep constitutional principles, to negotiate the Article 50 process in good faith, with the involvement of the devolved administrations.
Old and New
The second faultline is the old and the new. Traditionally, the British Constitution features the Queen-in-Parliament at its heart. There are no higher norms in the British constitutional order than those laws duly passed by the House of Commons and the House of Lords. What Parliament enacts is the supreme law of the land and Parliament can make or unmake any law on any subject. The prerogative continues to subsist as, in Dicey’s terms, the “residue of discretionary or arbitrary authority, which at any given time is legally left in the hands of the Crown”, as long as it has not been superceded by statute.
But this old view is, to some extent, being displaced by the new. On the one hand, there is significant momentum behind those who argue that sovereignty is “bi-polar”, shared between Parliament (acting in the political sphere by developing policy) and the courts (acting in the legal sphere by developing the common law), and those who argue that sovereignty, in today’s globalized world of international agreements and regulatory standards, is chimerical; moreover, with its patchwork of devolution statutes, Britain resembles less and less a unitary state. On the other hand, direct democracy has begun to creep onto territory previously occupied by Parliament. Referendums, whether on devolution of powers, reform of the electoral system or membership of international organizations, have become predicates to the passing of legislation on certain fundamental matters. Meanwhile, political leaders are increasingly elected by party members, not parliamentary caucuses, such that their mandates derive more from the general public than their colleagues. Finally — and here the two hands meet — the common law has begun to recognize a new category of ‘constitutional statutes’, which are immune from implied repeal and, beyond that, may be taken to express certain constitutional fundamentals about the British legal order. The upshot is that there is, arguably, a “hierarchy of domestic constitutional norms“, a phenomenon that can more readily emerge in a climate in which sovereignty is fragmented and/or variegated.
This prompts two questions in respect of Article 50. First, how should the growing importance of direct democracy be taken into account in the Article 50 process? One line of criticism of the Miller decision is based on how little weight was given to the referendum:
Indeed, the fact that ministers wish to trigger Article 50 early in 2017 because and only because they have been instructed to do so by the British people is overlooked in the judgment. This is a stark omission. Ministers’ exercise of the prerogative to trigger Article 50 is no ordinary executive act: it is an act ministers have been told to undertake in a referendum authorised by Act of Parliament.
Indeed, it has been argued that “The [Divisional] Court’s determination to examine the constitutional appropriateness of executive action triggering Article 50 without any reference to the broader context lends the decision a highly artificial air”.
However, the procedural posture of the Miller litigation has to be taken into account. The claimants do not attack the reasonableness of political reliance on the referendum result to justify sending the Article 50 notification. Rather, they attack the existence of a prerogative power to send the Article 50 notice in the first place: their claim goes to the jurisdiction to send the Article 50 notification, not to the merits of sending it. Indeed, pushed to its logical conclusion, the government’s response is that it would have jurisdiction to send an Article 50 notification with or without a referendum. Put another way, the government’s argument does not depend on the “new” constitution at all but relies rather on what Lord Reid described as “a relic of a past age, not lost by disuse, but only available for a case not covered by statute”. For this reason the Divisional Court gave no weight to the referendum.
By contrast, Maguire J. relied on the referendum because in the Northern Ireland case the reasonableness of the government’s exercise of the prerogative was in issue. Maguire J. rejected the claim that triggering Article 50 would be substantively unreasonable. A number of arguments were made, relating to matters such as Northern Ireland’s unique constitutional place in the United Kingdom, the need to assess alternative options and the giving of excessive weight to the referendum result (see para. 125). For Maguire J., it was “difficult to avoid the conclusion that a decision concerning notification under Article 50(2) made at the most senior level in United Kingdom politics, giving notice of withdrawal from the EU by the United Kingdom following a national referendum, is other than one of high policy…unsuitable for judicial review” (at para. 133, emphasis added). The referendum result has significant political weight and, in the right context, significant legal weight — but only in the right context.
Indeed, the referendum has so much political weight that even if substance trumps form in the new constitutional order such that even if the devolved administrations have a legal right to withhold consent or even veto the triggering of Article 50 they might decide not to do so: the Scottish government “has been careful since the EU referendum to argue that the will of the Scottish people to remain in the EU should be respected, not that this should override the desire of the majority in England and Wales to leave”. Here, the form/substance and old/new faultlines overlap.
Secondly, what role does the notion of a “constitutional statute” play? In Miller, the Divisional Court placed significant emphasis on the “constitutional” nature of the 1972 Act. The government’s argument that the claimants had to identify an abrogation of the prerogative in the 1972 Act (the operation of which is contingent on the government entering into agreements on the international plane: see paras. 77, 93(3)) “left out part of the relevant constitutional background” (at para. 84). Ratther, statutory interpretation — especially of a constitutional statute — “must proceed having regard to background constitutional principles which inform the inferences to be drawn as to what Parliament intended by legislating the way it did”; the statute has to be read “in the light of constitutional principle” (at para. 82).
This approach has been criticized because of the reliance by the Divisional Court on an intention imputed to Parliament. For instance, “the common law’s designation of a statute as ‘constitutional’ does not tell us anything whatever about legislative intention, because that designation is in the first place a matter of common law”. Put another way: “a statutory provision is constitutional not because the legislature intended it to have that status (which in any case had not been recognized in law when the 1972 Act was passing through Parliament) but because the common law confers that status on it”. The critics are surely right on this point: the notion of a statute being “constitutional” in nature only emerged many years after the enactment of the 1972 Act; Parliament could not be said to have “intended” effects that it has subsequently been interpreted as having. In any event, “a defence of Miller does not require us to accord a special legal status to the ECA”; a close reading of para. 93 of the Divisional Court’s judgment reveals that the “constitutional” nature of the 1972 Act actually played a very minor role in the analysis.
Once the language of legislative intent has been stripped away, the core of the Divisional Court’s point on statutory interpretation is that “the major constitutional importance” of the 1972 Act belied the argument that “Parliament nonetheless intended that its legal effects could be removed by the Crown through the use of its prerogative powers” (at para. 88). That the executive has a broad foreign affairs prerogative is so only because this prerogative cannot be used to modify domestic law (see paras. 89, 91). As a result, the “clear and necessary implication” of the provisions of the 1972 Act, read in their constitutional context, was to exclude the possibility that the legislation could be turned into an empty shell by the executive acting alone. Moreover, although the normative force of a “constitutional” statute comes from the common law — “the very reason EU law is sui generis as a matter of constitutional law is because domestic law has provided for its direct effect” — the normative force of a different order to anything else the common law has recognized: no other statute has been held to be immune from implied repeal or require the inoperability of another law passed by Parliament; that the 1972 Act, as passed by Parliament and interpreted by the courts is a “constitutional innovation” counts in favour of the Divisional Court’s decision. The result, it would seem, is not so much that the new trumps the old in the sense that constitutional statutes have developed an overwhelming gravitational force but that the new trumps in the old in the sense that the prerogative cannot be used to undermine one of the pillars of the contemporary constitutional order. There is a political analogue to this point: if the devolution statutes, unquestionably a ‘new’ part of the constitution, are to be altered fundamentally by Brexit, there will be much more scope on the political plane for the devolved administrations to insist that they ought to play an important role in the process of shaping Britain’s position in the Article 50 negotiations, in order to safeguard their constitutional acquis.
Legal and Political
The third faultline lies between the legal constitution and the political constitution. It might be said that there are (at least) two British Constitutions. One — the legal constitution — is primarily characterized by legal constraints and due process. For instance, it is “fundamental to the rule of law that decisions and actions of the executive are, subject to necessary well established exceptions (such as declarations of war), and jealously scrutinised statutory exceptions, reviewable by the court at the suit of an interested citizen” (R. (Evans) v. Attorney General,  UKSC 21, at para. 53). Similarly, constitutional principles will constrain grants of authority, such that powers to modify statutory provisions by subordinate legislation (so-called Henry VIII Clauses) will be narrowly construed. And, of course, where fundamental rights are at stake, Parliament must clearly authorise their elimination: this is the “principle of legality” and it “means that Parliament must squarely confront what it is doing and accept the political cost” (R. v. Home Secretary, ex parte Simms  2 AC 115, per Lord Hoffman).
But another British Constitution — the political constitution — is characterised by public accountability and the political process. As Lord Sumption recently put it in R. (Lord Carlile) v. Home Secretary  AC 945, at para. 32, in support of the proposition that a wider margin of appreciation should be available to elected officials whose decisions infringe protected rights:
…in a democracy a person charged with making assessments of this kind should be politically responsible for them. Ministers are politically responsible for the consequences of their decision. Judges are not. These considerations are particularly important in the context of decisions about national security…It is pre-eminently an area in which the responsibility for a judgment that proves to be wrong should go hand in hand with political removability.
In this Constitution, constitutional principles, including fundamental rights, are protected by the proper operation of political institutions and public debate:
Parliament has its own special means of ensuring that the executive, in the exercise of delegated functions, performs in a way which Parliament finds appropriate. Ideally, it is these latter methods which should be used to check executive errors and excesses; for it is the task of Parliament and the executive in tandem, not of the courts, to govern the country (R. v. Home Secretary, ex parte Fire Brigades Union  2 AC 513, per Lord Mustill)
In recent years, the legal constitution has tended to have the upper hand, for reasons Lord Mustill gave in the next sentence of his speech in Fire Brigades Union:
In recent years, however, the employment in practice of these specifically Parliamentary remedies has on occasion been perceived as falling short, and sometimes well short, of what was needed to bring the performance of the executive into line with the law, and with the minimum standards of fairness implicit in every Parliamentary delegation of a decision-making function. To avoid a vacuum in which the citizen would be left without protection against a misuse of executive powers the courts have had no option but to occupy the dead ground in a manner, and in areas of public life, which could not have been foreseen thirty years ago.
All that said, however, Lord Mustill was in the minority in finding for the Minister in that case.
In the MIller litigation, there is a clash between the legal and political constitution. It might be said from the perspective of the legal constitution that exercising the prerogative to trigger Article 50 would override rights by executive fiat, fundamental rights moreover that Parliament has expressly incorporated into domestic law by the 1972 Act. But it might with equal force be said from the perspective of the political constitution that the executive is responsible to Parliament, the prerogative must accordingly be given a wide scope and because s. 2(1) does not set out to limit the prerogative to enter into and withdraw from international agreements, it would disrupt orthodox constitutional arrangements to require legislation to authorize withdrawal from the EU. Moreover — and here the new constitution supports the political constitution — the executive is acting in pursuance of a referendum mandate that is accepted both by the government and Her Majesty’s Loyal Opposition.
In McCord, Maguire J. viewed matters from the perspective of the political constitution, “absent express provision being made, abridgment of the prerogative by a statute or statutory scheme must arise by necessary implication” (at para. 83). But in Miller, the Divisional Court looked at the arguments from a different viewpoint:
…the Secretary of State’s submission, in our view, glossed over an important aspect of [the] starting point for the interpretation of the ECA 1972 and proceeded to a contention that the onus was on the claimants to point to express language in the statute removing the Crown’s prerogative in relation to the conduct of international relations on behalf of the United Kingdom…the Secretary of State’s submission on section 2(1) of the ECA 1972 gave no value to the usual constitutional principle that, unless Parliament legislates to the contrary, the Crown should not have power to vary the law of the land by the exercise of its prerogative powers (at para. 84).
These starting points were, if not decisive, extremely important to the results reached in McCord and Miller.
One important component of the UK’s political institutions is worth mentioning at this point: the House of Lords. In form, the House of Lords has nothing more than a suspensory veto of legislation: a government determined to get legislation passed can invoke the Parliament Acts 1911 and 1949 such that a Bill will make it to the statutebook notwithstanding the refusal of the Lords to pass it. In substance, a suspensory veto would be extraordinarily powerful in respect of Article 50. It would delay the triggering of Article 50 by more than a year.
Would this be fair? In the political constitution, matters are regulated by convention rather than by law. The House of Lords exercises its power of suspensory veto by reference to the Salisbury Convention, pursuant to which it will assent to legislation countenanced in the electoral manifesto of a party elected with a majority in the House of Commons. But the Salisbury Convention does not bind the House of Lords in respect of matters not contained in an electoral manifesto: in the Conservative Party’s 2015 manifesto, there was a commitment to hold a referendum — and giving effect to the result can be seen as a logical next stop — but there was equally a commitment to remain in the Single Market — which would be imperilled by triggering Article 50 because, in the absence of an agreement within the two-year time period allowed for, there would be a ‘Hard Brexit’ in which the UK leaves the Single Market. Form and substance are important again here: is the giving of the Article 50 notice a mere administrative formality made inevitable by the referendum, or is it a policy-laden decision that should be accompanied by sustained parliamentary debate to outline publicly the government’s objectives for the negotiations with the EU? Here, substance may matter much more than form and the House of Lords might feel entitled to place substantive limitations, designed to guarantee continued membership of the Single Market (albeit perhaps as a temporary measure to ensure an orderly “Flexcit”), on the exercise of Article 50.
However, the force of the mandate from the referendum, an innovation of the new constitution, may overwhelm the supervisory mandate of the House of Lords, very much a feature of the old constitution. The new constitution may, in the circumstances, ultimately trump the old.
Principle and Practice
Fourth, principle and practice. In legal terms, the distinction between principle and practice influences how one interprets the available case-law. Departing the EU is not the first event that has prompted the British government to resort to the prerogative and it is not the first occasion on which dissentients have claimed that the prerogative is limited by statutory provisions or the common law. Previously decided cases, in which courts have sought to define the relationship between prerogative and statute, form an important part of the background context to the Brexit litigation. As all common lawyers know, however, determining the ratio decidendi of a case is a difficult interpretive exercise.
One view might be said to favour practice, looking to particular past cases in which the prerogative was held to be ousted by legislation to conclude:
The Crown may, by legislation under the prerogative, alter domestic law in the legal systems of the U.K. when doing so is not inconsistent with an Act of Parliament which occupies the juridical space…prerogative powers can be used to change domestic law in the U.K. if the power genuinely is part of the prerogative and if the change is not inconsistent with the requirements of an Act of Parliament which occupies the field in question…The question in each case is whether the whether the scope of the prerogative has been limited by Act of Parliament expressly or by necessary implication.
Of course, this analysis seeks to draw out a general principle (that the prerogative must be ousted expressly or by necessary implication) but does so by cleaving close to the decided cases, reasoning primarily by analogy. The result is that no clear language ousting the prerogative can be identified. Put differently, there is no precedent for the executive’s treaty-making prerogative being eliminated by legislation.
Another view might be said to favour the drawing out of more general principles, on which the decided cases are again closely parsed but with broader ambitions. So it has been said that the prerogative cases reveal a distinction between “abeyance” — situations in which the prerogative was indeed ousted expressly or by necessary implication — and “frustration” — situations in which an exercise of the prerogative would have frustrated the operation of a statutory provision:
The first principle could be labelled ‘the abeyance principle’. It means that where statute and prerogative directly overlap, the prerogative goes into abeyance. Essentially, the statute replaces the previous prerogative power, which disappears from the scene and can no longer be used. The second principle is ‘the frustration principle’. It means that where a statute and prerogative could be inconsistent, without overlapping, the prerogative cannot be used inconsistently with the intention of parliament as expressed in the relevant statute. The crucial distinction is that in the first category, the prerogative is no longer available to the Crown at all. In the second category, the prerogative is still ‘live’ and available to the Crown – it simply may not be exercised in a way that is inconsistent with parliament’s intention.
The result is that although the prerogative is not held in abeyance by the 1972 Act, its exercise to as to withdraw the United Kingdom from the Treaties would frustrate the 1972 Act. It might be said in addition that this view ultimately favours substance over form and, perhaps, the new constitution over the old.
In the political sphere, the faultline between practice and principle can also be observed. The underlying question is the extent to which Parliament should be involved in pulling the Article 50 trigger and overseeing the negotiations prior to Britain’s departure from the EU. Strictly speaking, close parliamentary oversight of the negotiation of a treaty — or indeed the exercise of the foreign affairs prerogative — would be unprecedened as a matter of practice. But it is not clear that practice should inevitably trump principle:
As the institution that is supposed to serve as the representative voice of the people, Parliament is given a central role in the UK constitutional order. It would be compatible with its sovereign status for it to have the final say as to whether and when the Article 50 trigger is pulled – and, by extension, it would enhance the democratic legitimacy of this decision.
More ambitiously, it might be possible to draw a general principle about the role Parliament can play in authorising and supervising executive action under the prerogative from recent parliamentary practice:
There is no past practice of the use of the prerogative power to initiate Article 50. Nor is there any past practice of a requirement of parliamentary deliberation prior to the use of the prerogative to leave an international Treaty. However, it may be possible to argue that a new convention has arisen by analogy with the convention, recognised in the Cabinet Manual, although subject to different interpretations, of providing the House of Commons with the ‘opportunity to debate’ before the commitment of troops to military action. Arguably, this convention has since evolved into a requirement for a parliamentary vote in favour of military action, following the failure to obtain a vote in favour of military action in Syria in 2013, the prerogative power then being exercised following a later vote in favour of military action in 2015.
The extent to which those who wish to hold the government to account can succeed in making this argument of principle will determine — regardless of who wins and loses in the courts — the extent of parliamentary oversight of the Brexit process.
My goal here has not been to predict the outcome of the Brexit litigation or the Brexit process, but simply to identify the legal and political faultlines exposed by Brexit. The outcome of the litigation and the Brexit process more broadly will depend on how the tectonic plates of the British Constitution shift along the faultlines of form and substance, old and new, legal and political, and principle and practice.