From what I understand, article 50 of the Lisbon treaty requires any state leaving to do so according to their domestic law.

It seems to be well understood that any form of hard border in Ireland is in breach of the Good Friday agreement, hence the backstop.

As the GFA is part of UK law, does it not therefore follow that a no-deal situation is illegal, as it would be in breach of that, and therefore invalid?

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    I would say that this properly belongs in law.stackexchange.com, as it's more of a legal question than a political one.
    – Dan Scally
    Commented Oct 2, 2019 at 9:39
  • Johnson is still committed to the 31st leaving date as he is expected to say in his speech at the Tory Conference today. Any sensible reading of the current state of UK legislation (The Benn Act) makes it clear that unless the EU refuses an extension, this would be illegal. Therefore we can only conclude that Johnson doesn't care about the legal position. Including whatever it might say in the GFA.
    – Jontia
    Commented Oct 2, 2019 at 9:43
  • @Jontia ...or he has a trick up his sleeve. Commented Oct 2, 2019 at 9:59
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    I am confused. Why do you think that it would invalid if illegal? If Johnson does not negotiate a treaty with the EU before October 31st he might well be in breach of UK law, but the UK would outside of the EU all the same. Commented Oct 2, 2019 at 10:53
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    @Caleth So I'm not a lawyer, but it seems to me that part of the procedure has already been concluded with the Article 50 notification, which was indeed done according to constitutional requirements. But I guess this is an academic discussion, of no great importance in practice. Commented Oct 2, 2019 at 12:44

2 Answers 2


I believe the "complication" mentioned in answer by user is actually the main issue here.

  • When the Supreme Court ruled on an issue between the Parliament, HM Government, and (technically) the Crown, it was the UK Supreme Court which had the final say.
  • Should the EU27 acknowledge the non-extension of Brexit, then as far as the EU27 are concerned the Brits are out after Halloween. Any opinion from a non-EU court after that date is about as legally binding on the EU as some ISIS guy crowning himself Caliph of the world.
  • The only way around that might be if the UK supreme court decides before Halloween to send an extension request in the name of the UK, and if the EU27 decide to accept that at face value (cf this question).

Of course nobody could stop the EU27 from re-admitting the UK and pretending that they never left. But that would be a political question, not a legal one.


Not just the Good Friday Agreement, but now also the Benn Act makes leaving without a deal on 31st October, without the refusal of the EU to grant an extension, illegal.

The question of if that would make it invalid is less clear. A court could possibly rule that due to the breech of the law it was invalid and therefore should be considered to have not happened, similarly to how the recent proroguing of Parliament was undone. However, this is mere speculation and complicated by the fact that the EU may consider the UK to have left after the 31st of October even if the UK doesn't.

In practice I think it is unlike to come to this, as any legal action would likely be concluded before October 31st and if leaving without a deal was considered unlawful the remedy would be to either seek an extension from the EU or withdraw Article 50 before that date.

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    The Benn act requires asking for an extension if No Deal is possible on the 19th. It still allows No Deal if the EU does not agree to an extension.
    – Caleth
    Commented Oct 2, 2019 at 12:30
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    Even in the case of the EU agreeing an extension, there might still be a no deal after the expiration of that extension, or if Parliament rejects a proposition by the EU for an extension past January 31st. The Benn Act in no way makes a no deal Brexit illegal.
    – Dan Scally
    Commented Oct 2, 2019 at 12:32
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    @Jontia I don't think your edit really improves the answer. Nothing in the Benn Act makes the fact of leaving without a deal illegal; the answer is simply wrong on that point. Indeed it could still be the choice Parliament makes now that they've claimed the right to veto a proposition from the EU on a longer extension. What would be illegal is the failure of the Prime Minister to request an extension, that's all.
    – Dan Scally
    Commented Oct 2, 2019 at 14:16
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    @user perhaps. Under the Fixed Term Parliament Act, it's not clear if Parliament can choose a new PM without a general election, and even if it can, that the person they choose would revoke the Article 50 notification, or how the ECJ would rule if it were revoked then re-issued. Also you are optimistic that there is "a larger plan"
    – Caleth
    Commented Oct 2, 2019 at 14:21
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    @Caleth the explanatory notes accompanying the Fixed Term Parliaments Act make it clear that the intention of the 14 day period between a vote of no confidence and a subsequent vote of confidence is to allow a new government to be formed without a general election. See the notes, Clause 2, section 16. The intention is to provide an opportunity for an alternative Government to be formed without an election.
    – Dan Scally
    Commented Oct 2, 2019 at 14:26

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