Last week saw the final submissions in the judicial review on whether the UK Government can formally trigger Brexit without parliamentary backing. Although dismissed by some, including the Government's own lawyers, as an attempt to "reverse Brexit", the Claimants' case asks a question that goes to the heart of the relationship between the Government, the Parliament, the courts and the people of the UK. Indeed, the outcome of this legal process may well have profoundly political implications.

Last week, the High Court concluded its hearing of the judicial review claim seeking a declaration that the UK Government does not have the legal power to trigger the Brexit process by giving notice under Article 50 of the Treaty on European Union without prior statutory authorisation from Parliament (R (Miller and Ors) v Secretary of State for Exiting the European Union).

It is unsurprising that the case has received so much attention. On Tuesday 18 October 2016, the submission by James Eadie QC, on behalf of the Government, that the Article 50 withdrawal agreement would "very likely" need to be approved by Parliament appeared directly to affect currency markets. Former Attorney General Dominic Grieve QC has also suggested in recent days that, if the Government loses the case, this would be likely to frustrate the Prime Minister's hope of triggering Article 50 by the end of March 2017.

Although dismissed by some, including the Government's own lawyers, as an attempt to "reverse Brexit", the Claimants' case asks a question that goes to the heart of the relationship between the Government, the Parliament, the courts and the people of the UK.

This blog does not seek to summarise all the arguments put forward by both sides. Instead, it focusses on the central conflict between two cornerstones of the UK constitution:

a) the principle of Parliamentary sovereignty, that Parliament has the power to make and un-make any law and no body or person has the right to override or set aside the legislation of Parliament; and

b) the royal prerogative powers, the centuries-old, residual powers of the Ministers of the Crown to, among other things, conduct the UK's international affairs on her behalf without the consent of Parliament.

You can find the 582-page transcript of proceedings here.

Finding meaning from silence

The origin of the case is the EU Referendum Act 2015, which gave rise to the EU Referendum itself (the "2015 Act"). Crucially, the 2015 Act does not specify the process by which the result of the referendum should be implemented in the event of a "Leave" vote. Both the Claimants and the Government agreed that this silence is significant, but for different and conflicting reasons.

The Claimants argued that the lack of a prescribed process for implementing the result of the referendum was due to the fact that Parliament intended for the EU referendum to be "advisory only". They argued that Parliament cannot be bound by the result and that, to the extent that implementing the will of the people would inevitably lead to changes in domestic law, only Parliament can lawfully do so.

The Government argued that Parliament knew when enacting the 2015 Act that the Government intended to give effect to the result of the EU referendum, whatever it may be, and that triggering Article 50 would be the only legal mechanism for leaving the EU. It was therefore on this understanding and expectation that Parliament passed the 2015 Act and the people voted on 23 June. Therefore, the Government contended, a lack of a prescribed procedure in the 2015 Act demonstrated that Parliament intended for the Government to be free to exercise its prerogative power to give effect to the will of the people by triggering Article 50, just like it does when negotiating other international treaties. The Government noted that it would be "a surprising submission in a modern democratic society" to argue, as the Claimants did, that it was "constitutionally appropriate for the British people to vote to leave and for Government and/or Parliament then to decline to give effect to that vote".

Is triggering Article 50 the point of no return or just the start of a process?

The Claimants' case necessarily proceeded on an interpretation of Article 50 that was not contested by the Government, namely that triggering Article 50 must be a unilaterally irrevocable and unconditional act. This was central to the Claimants' argument because it was the basis on which they argued that, once Article 50 is triggered, it is inevitable that the UK will leave the EU, the EU Treaties will cease to apply to the UK and the legal rights enjoyed by UK citizens as a consequence of the UK's EU Membership would – without a subsequent intervention by Parliament – be lost. The Claimants contended that, because of the inevitable impact of triggering Article 50 on domestic law, only Parliament can decide to trigger it.

The Claimants acknowledged the Government's plans to introduce the Great Repeal Bill, which is intended to preserve, where possible, many of the rights in domestic law that would otherwise be lost on the day of Brexit. However, the Claimants argued that the Government could not rely on the Great Repeal Bill as a defence to the claim; the question here wasn't whether these rights would in fact be preserved (which would be a question for Parliament in due course), but whether the Government had the right to take them away in the first place.

The Court correctly identified that, if, contrary to the Claimants' case, an Article 50 notice could be unilaterally revoked, the Claimants' case would be "blown out the water". It was therefore significant, albeit not surprising in the circumstances, that the Government decided not to contest this point. Significant because it would have made its defence a lot easier; unsurprising because it was clearly politically unpalatable for the Government to argue before the Court that it could retract its Article 50 notification, once given, or for the true interpretation of Article 50, which is ultimately a question of EU law, to have to be determined by the Court of Justice of the EU.

This uncontested understanding set the stage for the Court to consider the crux of the case: can the Government use the royal prerogative to do an act on the international stage that would inevitably, albeit not immediately, affect its citizens' domestic rights?

Between a bedrock doctrine and a hard principle

The UK is a dualist legal system, meaning that international agreements entered into by the Government on behalf of the UK do not form part of domestic law unless and until Parliament incorporates them by domestic legislation. This is why Parliament enacted the European Communities Act 1972 (the "ECA"), which gave effect to the new relationship between the (then) European Economic Community and the UK, when the UK first joined.

However, the Claimants argued (on the basis of cases such as Van Gend en Loos and Simmenthal) that the EU Treaties are unlike other international treaties because, once they were incorporated into UK law, they created a "new legal order" that conferred rights on UK citizens "which [have] become part of their legal heritage". They argued that these rights were made part of domestic law by the ECA, and other statutes, and that triggering Article 50 would necessarily take them away.

They went on to contend that the Government's general prerogative power to conduct international affairs on behalf of the UK without prior Parliamentary approval existed in such broad terms precisely because its actions on the international stage did not normally have any domestic legal effect. They cited a number of cases (including the Case of Proclamations, International Tin Council, Laker and Fire Brigade's Union) in aid of their proposition that, in the words of Lord Oliver in International Tin Council:

… as a matter of the constitutional law of the United Kingdom, the Royal Prerogative, whilst it embraces the making of treaties, does not extend to altering the law or conferring rights upon individuals, or depriving individuals of rights which they enjoy in domestic law without the intervention of Parliament.

Therefore, the Claimants invited the Court to determine that the Government did not have the power under the royal prerogative to do an act on the international stage (ie trigger Article 50) that would inevitably result in the removal of rights that Parliament had granted. If the Court agreed with the Claimants, this would mean that the Government could only trigger Article 50 after Parliament had passed legislation granting it the power to do so.

In response, the Government maintained that triggering Article 50 was self-evidently an act on the international stage and therefore one that the Government was entitled to take, unless Parliament had limited its power to do so. The Government disputed the Claimants' contention that international acts done through exercise of the prerogative could not lawfully affect domestic rights, arguing instead that the cases the Claimants relied on to make this point in fact did no more than establish the basic and uncontroversial principle that international treaties were not "self-executing" in domestic law (International Tin Council).

The Government did not dispute that Parliament could lawfully restrict, or even replace entirely, the exercise of the prerogative by statute. However, the Government argued that Parliament had not done so in this case. It claimed that the relevant statutes demonstrated that Parliament had in fact continued to consent to the Government exercising the prerogative to withdraw from the EU Treaties, even in the knowledge that withdrawal would have various legal effects in domestic law.

The Government cited De Keyser's Hotel to support its proposition that Parliament could restrict the use of the prerogative only by (a) express words or necessary implication, (b) enacting a statute that directly regulated the same matters, or (c) enacting a statute that regulated the "whole ground" with which the prerogative was concerned. It went further, arguing that "when Parliament wishes to fetter the Crown's treaty making power in relation to [EU] law, it does so in express terms" (per Rees-Mogg), but, if the Court did not agree, the Government said that it should at least accept the former and wider test as the correct one in this case.

The Government also argued that a number of the cases cited by the Claimants (Laker, Fire Brigade's Union) were, in fact, examples of the Government's proposition in action: cases where Parliament had enacted an elaborate code or statutory scheme intended to regulate matters that had previously been the subject of the prerogative.

After considering the various statutes enacted by Parliament to regulate the UK's relationship with the EU, including the ECA, the European Parliamentary Elections Act 1978, the European Union (Amendment) Act 2008 (the "2008 Act"), the European Union Act 2011 and the 2015 Act, the Government concluded that none of these statutes restricted the Government's use of the prerogative to trigger Article 50.

It claimed that this was because Parliament had intended for the prerogative to remain available in respect of triggering Article 50, arguing that Parliament can express its sovereignty as much by not doing something as by doing something. Thus, the Government argued it was entitled to trigger Article 50 without prior Parliamentary approval, even if this impacted on domestic rights, because this was Parliament's intention.

The status of referendums in the UK constitution

The Claimants claimed that the logical conclusion of the Government's argument (outlined above) was that it would have been possible for the Prime Minister to have "woken up one morning" and decided to trigger Article 50, thereby leading inevitably to the same removal of citizens' rights, even if there had been no referendum. This was because, the Claimants argued, the referendum result had no legally binding effect.

The Government's answer to this argument was not entirely clear. However, it raises an important constitutional question: what is the status of the referendum in the UK constitution?

The Claimants' position was based on the classical conception of the UK as a representative parliamentary democracy, as described by A. V. Dicey in his archetypal text on the British constitution, originally published in 1885, in which "the sole legal right of the electors [ie the people] under the… constitution is to elect a member of Parliament". Under this conception of parliamentary democracy, Parliament's will, as expressed in Acts of Parliament, cannot be challenged (at least in the courts), even by the clearly expressed will of the people.

However, since Dicey, the UK has held three national referendums, and a number of regional ones, on various issues. Is it still the case that the UK constitution "knows nothing of the people"? It could be argued that the use of referendums to decide matters of fundamental constitutional importance is itself contrary to the sovereignty of Parliament. Was the Government inviting the Court to decide that the referendum result had a special constitutional status that gave the Government a legal mandate to trigger Article 50 that it did not otherwise have?

It did not go so far as to say so expressly. However, Eadie QC did accuse the Claimants of seeking to persuade the Court to apply constitutional principles in a way that denied the flexibility at the heart of the constitution, as well as suggestively referring to the following statement by Lord Bingham in Robinson v Secretary of State for Northern Ireland, which Eadie QC described as a "statement of principle":

It would no doubt be possible, in theory at least, to devise a constitution in which all political contingencies would be the subject of predetermined mechanistic rules to be applied as and when the particular contingency arose. But such an approach would not be consistent with ordinary constitutional practice in Britain. There are of course certain fixed rules, such as those governing the maximum duration of parliaments or the period for which the House of Lords may delay the passage of legislation. But matters of potentially great importance are left to the judgment either of political leaders (whether and when to seek a dissolution, for instance) or, even if to a diminished extent, of the crown (whether to grant a dissolution). Where constitutional arrangements retain scope for the exercise of political judgment they permit a flexible response to differing and unpredictable events in a way which the application of strict rules would preclude.

What comes next?

The Court said at the end of proceedings that it would give its decision as soon as possible. It is almost inevitable that whoever loses will appeal. It is widely expected that the case will progress straight to the Supreme Court, who have reportedly set aside time to hear the case in December this year.

If the Government ultimately loses the case, it would need to obtain Parliamentary approval for triggering Article 50 in the form of an Act of Parliament.

Parliament is capable of passing legislation exceptionally quickly (the Anti-terrorism, Crime and Security Act 2001 passed through Parliament in less than a month), but in this case it seems likely that the passage of a Bill would be the subject of proposed amendments and debate about the process by which the Brexit negotiations should be conducted. Parliament could demand that any Bill includes detailed procedures for ensuring effective Parliamentary oversight of the negotiations.

The outcome of this legal process may well have profoundly political implications.