Does Article 50 require an Act of Parliament? A brief thought-experiment.

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3rd July 2016

Here is a question: would a decision by the United Kingdom to withdraw from the European Union require an Act of Parliament?

On this question, one week or so after the referendum vote for Leave, it may be fairly said that “views differ”.  Some constitutional lawyers say one thing, others say the other.

In favour of an Act of Parliament being the only way, see David Pannick in The Times, Nick Barber, Tom Hickman and Jeff King on the UK Constitutional Law Association site, and Adam Tucker at the same site.

In favour of it being just one means among others, see Carl Gardner at Head of Legal and Mark Elliott at Public Law for Everyone.

When the views of experts differ it is a temptation to just shrug with despair.

Nobody would blame you.


This post sets out a contention – based on a thought-experiment – that an Act would not be required.

An Act could be a means of the decision being made – on that point views do not differ – but it is not the only way.

What follows is that contention – but please note this post is a (minor) contribution on one side of a debate rather than an (attempt at) objective exposition of an area of controversy.


First, let’s remind ourselves of the relevant parts of the first two paragraphs of Article 50 (I have omitted some not relevant text):

1. Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements.

2. A Member State which decides to withdraw shall notify the European Council of its intention.

The key question is what is a “decision” in accordance with the UK’s “own constitutional requirements”.


Second, let’s consider a thought-experiment based on a counterfactual situation.

Think back to last Friday, the day the result of the referendum came in.

Consider now the following facts:

– a national referendum had taken place in accordance with a statute passed for the sole purpose of that referendum;

–  33,577,342 voters voted in that referendum (72% of the electorate);

– there was a clear (if narrow) vote in favour of Leave;

– the Prime Minister had said expressly at the beginning of the referendum campaign that “If the British people vote to leave there is only one way to bring that about – and that is to trigger Article 50 of the Treaties and begin the process of exit. And the British people would rightly expect that to start straight away”; and

– the government had expressly stated during the campaign “This is your decision. The government will implement what you decide”.

Taking these five facts, imagine now the Prime Minister (by himself or after consulting with Cabinet) taking the decision (or purporting to take that decision) on that Friday that the UK had decided to withdraw from the European Union in accordance with its own constitutional requirements.

There had been a special referendum passed by Parliament for the purpose, there had been a heavy turn-out and a decisive result in the vote in that referendum, and the government had stated clearly that the decision would be implemented and the Article 50 straight away.

Imagine now the Prime Minister that same Friday had been minded to notify the European Council of such a (purported) decision.

What would be the legal position if some wily litigant- lets call her Ms Haddock – had been quick enough to make a legal challenge that same day?

I cannot see how the courts would have seen the claim as justiciable: there had been a special referendum with a clear result on a high turn-out and the government had moved to implement the decision straight-away as it clearly stated it would do.

Even if the court had allowed Ms Haddock’s claim to be heard, I cannot see how it would have quashed the decision or injuncted the Prime Minister from notifying the European Council.

Not only would there have been an uproar as the courts blocked the popular decision (and significant economic instability), one suspects the courts would have said that to interfere in the decision and the notification would be a violation of the separation of powers.

Our litigant Ms Haddock would have lost.

In other words: for the Prime Minister (with or without cabinet) to make such a decision and notification would not be unlawful.  It would thereby be lawful.


The thing about questions of law is that the legal position on day one should normally be the same as the legal position on day ten, if the law and the facts are the same.

If so, then if the Prime Minister today – or in September, or whenever – took the same decision and made the notification then it would be just as lawful on day ten than on day one (assuming the law or facts have not changed).

As a normative view, I agree that an Act of Parliament (or a resolution or motion in Parliament) should be the method: the referendum was not legally binding – whatever the government said – and such a significant decision should be made by Parliament.

A vote by Parliament would also provide convenient cover for a government reluctant to press the Article 50 button.

(It may well be that the legal position changes, and Parliament passes a resolution or motion that the final decision has to be made by itself.)

But I cannot see how a decision on notification made by the Prime Minister on the day of the referendum result itself would have been quashed or injuncted by the courts, had Ms Haddock brought a claim to block it.

If that was true on that day, that would be true now, all other things being equal.

And if this is correct, then presumably an Act of Parliament is not the only means by which the Article 50(1) decision can be lawfully made.


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27 thoughts on “Does Article 50 require an Act of Parliament? A brief thought-experiment.”

  1. I hear what you say, but if the Pannick/Barber/Hickman/King argument is correct, none of those 5 facts are relevant.

    Your argument appears to rely upon some form of legitimate expectation held by Leave voters placing a duty on the PM to serve the A50 notification. You’d truly struggle with that (beginning with identifying persons with standing, given the secrcy of the ballot), even were serving the A50 notification under prerogative power permissible.

    But in any event, a legitimate expectation held by a body of voters cannot legitimise an illegal act; whether immediately after the event said to trigger that expectation, or subsequently. The genral population has an expectation that whatever representations a government makes, it will act within the law – or be held to acount for it.

    1. My contention (not an argument) does not rely on legitimate expectation. Had it done so, I would have mentioned it. So your comments which follow (“truly struggle” blah, for two paragraphs) are based on something I did not say. Straw-dolly stuff. Sorry, Robin!

        1. That was a serious question; what *is* (in your view) the relevance of the five facts you mention, if not to seek to establish a legitimate expectation (and assuming you’re not saying that they allow one to gloss the statute)?

  2. Whether the (effectively English) PM alone can trigger an Article 50, or whether an Act of Parliament is required overlooks one political problem, even if the legality of either is in no doubt.

    Does the PM alone, or a parliament in which an English party holds the majority, have the (legal) authority to override the devolved institutions? If yes, then isn’t devolution a bit of a sham; if no, and there is no regional agreement, how can the PM or parliament actually proceed?

  3. Is it the case that in recent times a ‘convention’ has arisen that Prime Ministers do not exercise prerogative – e.g. to declare war; to instruct the RAF to bomb Syria – but, rather, seek Parliamentary consent; and if it is the case, is this ‘convention’ relevant here?

    1. Arguably so on proposition one, however you might struggle to submit that such a recent “convention” was a convention at all.

      Second potential counterargument is that the point of going to Parl before exercising prerogative powers in those cases has been so the act reflects the will of the people (via their representatives) – where you have a referendum result telling you just that, that rationale falls away.

  4. The missing ingredient in this thought experiment seems to me the question whether Parliament knew about the PM’s promise to invoke Article 50 in case of a Leave result, BEFORE its vote on the referendum law. If it did, then it intentionally delegated its authority to the referendum voters. But if the PM made that statement after the law went through Parliament, no parliamentary authority could have been transferred to anyone else, and Parliament would still have to vote on Article 50 notification.

    1. “If it did, then it intentionally delegated its authority to the referendum voters.”

      Not so; by not including any binding language in the statute, it ensured that the statute wasn’t binding – irrespective of what the PM said, whether inside or outside Parliament.

      1. Making the statute binding would have made Article 50 invocation inevitable after the referendum. This discussion is clearly not about that counterfactual case, but about the question whether the actual, non-binding language could have led to a court squashing invocation. Why would the circumstances of legislation be irrelevant to a court? Aren’t they supposed to consider the intention of a law in addition to its literal content, particularly when the latter doesn’t provide a straight answer?

        1. Its literal content gives a very clear answer – there was no intent for the referendum to have any legal effect whatsoever. Pepper v Hart only allows reference to proceedings in Parliament as to legislative intent where there is ambiguity on the statute’s face (broadly speaking); otherwise no such reference is permissible.

    2. In point of fact, no promise was made to invoke Article 50 by prerogative. In Parliament, the ministerial statement that was made to the House of Lords was quite frank about the fact that whilst the Government would respect the result of the referendum, it did not know what would be the mechanism to be followed after a “Leave” vote. To quote from Hansard (HL Deb 13 October 2015, vol 765, col 221):

      “The Bill is all about putting the question to the British people. It does not make provision about what happens next. I was asked whether the result would be legally binding. Clearly, at the moment, it is not sensible for us to guess about the best way to implement the result, but, as the noble Lord, Lord Hannay, said, this would be the first time that a member state had had the opportunity to vote to leave. If we got to the position where the country decided that it wished to leave, we would then get into the newer territory of working through those procedures.”

      1. I think, while interesting and helpful, this particular partial reference by TT Arvind to the Hansard proceedings in the passage of the Bill is not determinative because, notwithstanding Pepper and Hart, the Courts have sole jurisdiction for judging what is “legally binding.”

        I think then Secretary of State for Foreign and Commonwealth Affairs Philip Hammond’s statement at second reading in the Commons that “the decision about our membership should be taken by the British people, not by Whitehall bureaucrats, certainly not by Brussels Eurocrats; not even by Government Ministers or parliamentarians in this Chamber” is consistent with the construction that the 2015 Act provided for a statutory decision by those enfranchised to vote in the referendum on the question of whether the UK should leave the EU. (Hansard 9 Jun 2015 : Column 1052).

        The tension between this interpretation and the remarks in the Lords could be resolved by saying Ministers did not understand the full legal implications of this statutory provision.

        The remarks in the Lords could be read as merely supporting the interpretation (almost certainly correct unless the Supreme Court gives new guidance on the constitutional status of referenda) that a statutory decision to leave the EU in the referendum could be revoked, in theory, by a subsequent Act of Parliament.

        This interpretation may lead to the Supreme Court considering the question of whether a statutory decision in a referendum to leave the EU, as evidenced by the Supreme Court’s own judgment, is sufficient to trigger Article 50 without notification, if it hears leapfrog appeals in December. After all, such a judgment incorporating this interpretation would make it finally evident to the world that the UK had decided to leave the EU.

        This would leave undetermined the question of whether a theoretical subsequent domestic Act of Parliament could halt the European Article 50 procedure once it had been triggered. Resolution of that question would appear to be within the sole competence of the Court of Justice of the European Union.

  5. David, I like this approach to the issue. But here’s an additional thought:

    We can run the same thought experiment works backwards rather than forwards. If a Prime Ministerial decision would have been lawful the day after the referendum, then it would have been lawful *before* the referendum as well. So if a prerogative decision to withdraw from the EU was lawful last Friday, then it would also have been lawful last year. And bear in mind that this kind of decision would be made by the Prime Minister, unilaterally, without further scrutiny and (because of the substance of Article 50) irreversibly. I think it would surprising if our membership of the EU has always been hanging by that thread.

    1. Thank you Adam.

      Just to be clear: do you think that a court that Friday would have quashed a decision and/or injuncted the prime minister from notifying the European Council?

      1. Yes, the UK’s treaties hang by a prerogative thread.

        Q: How does the UK enter into or abrogate any treaty?
        A: Through the exercise of prerogative powers.

        Q: Who exercises those powers?
        A: Formally the Queen, but practically through the decisions of government ministers and the actions of UK civil servants negotiating their terms.

        The FCO reckons the UK has entered into at least 14,000 treaties since 1834 – – but very few were ever explicitly approved by Parliament. (Bonus points to those who recall the Anglo-Portuguese Commercial Treaty Acts of 1914 and 1916; double points and a bottle of fortified wine to those who know what they are about).

        Since 2010, treaties must be laid before Parliament before they are ratified, but some become effective on signature. Previously, they were laid before Parliament under the Ponsonby Rule announced in 1924. (See )

        Clearly the “decision” made by the government to leave the EU will have wide-ranging political and legislative consequences, not least the European Communities Act 1972 but over 40 years worth of statutory instruments, but I can’t see a reason why government should not formally make the decision to leave and notify that decision if to the European Council.

        Surely the decision and/or notification (or lack of either) are political matters. Are they justiciable in any event? I am sure the arguments will be fascinating, but the courts are going to struggle to determine that the law requires the government to exercise or not exercise prerogative powers to make or abrogate any treaty.

      2. If he had woken up on Wednesday the 23rd and decided to trigger it then would a court have quashed it?

        I’ve never been in a court in my life so I don’t no either way. But it seems strange that membership of the EU has been at the PMs whim since the Treaty of Lisbon

        1. There are many possibilities inherent in the UK’s rather ill-defined and largely ad hoc (some would say evolving and flexible) constitutional arrangements.

          The Queen could decide tomorrow to dismiss the Prime Minister, or dissolve Parliament, on a whim. She won’t (and if she did without reason, I expect she would lose the possibility of doing it ever again in short order). Similarly, the Prime Minister is not going to randomly abrogate any of the UK’s treaties on a whim, even if theoretically he could exercise the royal prerogative to do so.

          In the case of the EU treaty, I suspect a random notification under Article 50 before the referendum would have been rejected as not being in accordance with the UK’s constitutional requirements, whereas any kind of formal notification after the referendum will be accepted.

  6. An interesting thought experiment, and one that all of us have pondered in alcohol-fuelled ‘if only’ conversations; and the legal laymen among us, myself included, have been relieved to see that Article 50 remains inactive.

    The bar-room consensus is that both Parliament and the courts would’ve challenged an executive decision to give Article 50 notification; and it would be moot because, on the day, Strasbourg and Brussels would’ve accepted any communication from London – even a cow with “We’re leaving” spray-painted on the side and led there by the Editor of the Sun – as a valid, binding, and irrevocable start to the Article 50 exit process.

    Would it be possible to defend this executive act – the Prime Minister, not the cow, and no spray paint – in court by arguing that an ill-thought-out and destructive act of dubious legality is, by its very nature, entirely in keeping with the expectations and conventions of Westminster?

  7. If Ms Herring went to court on the day after the result on the basis that Article 50 had not been triggered when it should have been, would she win? If she didn’t win, would that mean it would always remain the case that such an action would never win, even years later if it still hasn’t been triggered?

  8. The thought experiment does not include the terms of existing statutes. Those include numerous specific provisions restricting the prerogative treaty-making power concerning amendments to the EU treaties. For example, the European Union (Amendment) Act 2008 and the European Union Act 2011 require an Act before amendments can be made. Terminating a treaty in its entirety is certainly an amendment of all of its provisions.

    Delivering an Article 50 notice is a a termination of the treaties effective 24 months after its date, unless accelerated by agreement. In respect of many of the affected treaty terms and the large number of U.K. statutes that would be rendered functors by that, the delivery of the notice amends statutes and abrogates the powers of Parliament. The executive cannot use the prerogative powers to do that. Barber, Hickman and King are correct in their analysis.

  9. One further comment: As an admirer of the late Tom Bingham and his best little book, “The Rule of Law”, I have wondered what of Brexit would concern him.

    The impact of leaving on human rights for one. The European Convention on Human Rights was implemented in the United Kingdom by the Human Rights Act 1998. Is this one of the European statutes that are to be repealed under the direction of the referendum? After nearly 45 years, vast amounts of the EU treaty provisions, decisions of the European Court of Justice, administrative directives of the European Commission etcetera are now inextricably part of the rule of law in the UK. These affect peoples lives in many, often beneficial, ways every day. They are needed and if they are accidentally erased, people will be surprised and may be harmed.

    It is a dangerous oversimplification to deal with this as a simple Remain/Leave choice. Unscrambling such an omelette is no small thing. To the annoyance of some of the political actors, Brussels will rule from the grave for many years after any exit.

  10. Another interpretation is that, because EU law and EU rights are so embedded into the UK’s legal system, the UK simply has no constitutional procedure for withdrawing from the EU. This would imply that Parliament would need to create one via primary legislation.

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