A full panel of all 11 Supreme Court judges will hear the Government’s appeal against the High Court’s ruling of 3 November 2016 which held that the Government could not proceed using royal prerogative powers to trigger Article 50 “in accordance with [its] constitutional requirements” (Article 50(1)) and that only Parliament had the power to make the notification. All parties should welcome the fact that the matter will be finally determined swiftly (5-8 December 2016 has been set aside for a hearing, with judgment expected by early January at the latest) by the highest court in the land.

Yet the Supreme Court may also have to decide – and quickly – whether it needs the assistance of the Court of Justice of the European Union (‘CJEU’) to interpret Article 50 correctly before it delivers its definitive judgment on the pending appeal.

Throughout this case, both sides presented their arguments on the basis that Article 50, once triggered, is irrevocable. It is also clear from the judgment that the High Court proceeded on this basis. However, there is considerable doubt as to whether Article 50 is in fact irreversible – the provision itself is silent on the point and Lord Kerr, one of the people involved in drafting it, has expressed the contrary view. In his opinion, Article 50 is not an absolute mechanism – there is scope for arguing that a State could unilaterally halt or even reverse the process once started. At its limit, the threat of so doing would be a considerable negotiating strength in seeking to persuade 27 sovereign countries of the necessity to extend the two year period set out in Article 50. It is therefore perhaps surprising that the Government has taken the position it has.

This uncertainty should be of concern not just to the UK, to Remainers or Brexiteers, but also to the EU and the other 27 EU countries. It is in everyone’s interest that we have swift and definitive resolution both on the application of Article 50 (i.e. by an EU court over an EU provision) and subsequently by the Supreme Court over the interpretation of the UK constitution (i.e. by the highest UK court over a UK issue).

For Remainers and perhaps those hoping for Parliamentary approval of a particular negotiating strategy aimed at retaining access to the single market, this should be of paramount importance, not least because whatever its negotiating stance, it is not in the UK’s gift to guarantee any kind of agreement. In other words if any Member State or the European Parliament chose to block an agreement with the UK, the UK would simply cease to be a Member State on the expiry of two years. For Brexiteers who are keen for the Government to press on with the UK’s exit plans, certainty is also key. A definitive answer to the question could be a guarantee that the Government would not be able to renege on the withdrawal process, or if Article 50 is revocable, a call to put plans in place to hold the Government to account.

Contained within the High Court’s judgment itself is a sense that the answer to the UK’s constitutional question cannot and should not be divorced from a proper understanding of the impact of triggering Article 50. As discussed, it was common ground between the parties that a notice validly given under Article 50(2) cannot be withdrawn. It is surprising that the Government’s position was to accept this – an uncharitable conclusion might be that this was precisely to avoid the necessity of a reference to the European Courts…

But this position is critical because its effect is to make the invoking of Article 50 the act that will “inevitably result in the complete withdrawal of the United Kingdom from membership of the European Union …at the end of the two year period” (judgment, paragraph 11). As such a withdrawal is itself incompatible with an Act of Parliament (i.e. the European Communities Act 1972 (‘ECA’)) and the High Court therefore ultimately concluded that as a matter of British constitutional law only Parliament can take a step which amends rights and obligations contained within an Act of Parliament. In short, this is the central basis for concluding that the Government may not act unilaterally under the Royal prerogative and must do so only with Parliament’s explicit consent.

Given this, it is somewhat troubling that this central tenet is one of ‘common ground’ rather than proper judicial scrutiny. Whether or not Article 50 is irrevocable is of course a matter of EU law and would therefore be a question for the CJEU’ to decide following a reference from the national court under the Article 267 procedure. Perhaps the first decision facing the Supreme Court should therefore be whether the revocability or otherwise of an Article 50(2) notice is necessary in order for it to reach its (sovereign) decision on the correct interpretation of the UK constitution. If it is the Supreme Court must make a reference to the CJEU unless the position in respect of EU law is clear (a so-called ‘acte clair’). Given the degree and extent of the debate on this issue, both in the UK and the EU, it is difficult to believe that the Supreme Court could ever arrive at a conclusion that the position was clear should it conclude that resolving this issue was necessary.

Of course it could be argued that the question is not necessary as it does not have any bearing on whether or not the Government can use its prerogative powers. Indeed some commentators have pointed out that there are other reasons that triggering Article 50 would not necessarily result in exiting the European Union, such as fundamental renegotiation of the Treaties whilst negotiations are ongoing. But there is no doubt that it forms an important part of the High Court’s judgment.

Indeed, if it were plain that an Article 50 trigger could be subsequently withdrawn, there would be a credible argument that notification alone is not incompatible with the ECA, because the Government would in practice be able to proceed on a conditional basis: notification would be given with a commitment to bring the negotiated outcome to Parliament before expiry of the two year deadline. Failure to agree an extension of this deadline or failure to obtain Parliamentary approval would require unilateral withdrawal of the Article 50(2) notice, all in accordance with the UK’s constitutional requirements and consistent with the wording in Article 50(1).

It is plainly essential not only that these issues are resolved, but resolved definitively and swiftly. Just as the UK judiciary has shown it is capable of moving very fast in arriving at the High Court judgment and subsequently in scheduling an appeal direct to the Supreme Court, so the Court of Justice has an expedited procedure which could (and should) be used to determine the position on Article 50. That should be of interest to the UK Government whose negotiating position would be significantly strengthened should the CJEU side with the view that Article 50(2) is a notice that can be given and withdrawn unilaterally by a Member State (conversely, a contrary view would put the UK in a worse position than it appears to accept today). But it should also be of interest to the EU and other Member States in seeking clarity over a provision that has the potential to stifle the negotiations for some considerable time.

Yet despite the above, it seems highly unlikely that a reference will be made, leaving the question of whether Article 50 is or is not reversible for another day. This would be the case should the Supreme Court determine that knowing the answer to this question is not essential to their judgment – for example, even assuming that notification could be unilaterally withdrawn, the Supreme Court may decide it is not enough to rely on the fact that Government could or even should withdraw notification prior to expiry of the two year notice period if Parliament has not approved the final exit deal. If, for whatever reason, the Government chose not to exercise its prerogative at that point, the UK would cease to be a member of the EU and those EU rights enshrined in the ECA would fall away. Hence, it may be argued, it is imperative that Parliament alone triggers a process which could ultimately result in the removal of rights contained within an Act of Parliament (i.e. the ratio of the High Court judgment).

What is clear is that we do not have long to wait to see what decisions are ultimately taken. Whatever the outcome, it would be somewhat of a surprise if they were not however taken by the Supreme Court alone.