We now know that the EU is willing to extend the date on which the UK leaves it, at least for a short period. But eventually, one of three things must happen: the UK can leave the EU with a deal, leave with no deal, or not leave.
That third option would involve the UK revoking Article 50 – the section of the Lisbon Treaty which, having been triggered on 29 March 2017, started a two-year countdown to leaving.
A petition calling for that option had been signed by more than three million people by Friday lunchtime.
How could Article 50 be revoked?
The European Court of Justice (ECJ) ruling last year confirmed that the UK could revoke Article 50 itself, without having to ask the other 27 EU countries for permission.
It said that if it did, the UK could remain a member of the EU on the same terms including keeping its budget rebate.
But it did set some conditions.
The ruling said revocation should be “unequivocal and unconditional”, suggesting that the UK could not simply revoke Article 50 in order to buy more time and then resubmit it at a later date.
Separate legal advice, from a top lawyer at the ECJ, said “appropriate legal instruments” could be used “if a Member State engaged in an abusive practice of using successive notifications and revocations in order to improve the terms of its withdrawal from the European Union”.
Theresa May or Parliament?
On this, the ECJ did not give a definitive answer. The court ruled that: “Revocation must be decided following a democratic process in accordance with national constitutional requirements.”
The UK government can use the power known as the Royal Prerogative, which allows it to do certain things including deploying armed forces, granting honours and altering international treaties without consulting Parliament.
So it is possible in theory, but unlikely, that Mrs May would be able to revoke Article 50 without giving MPs the chance to vote on it.
She has no plans to do so though, saying: “I do not believe that we should be revoking Article 50.”
There are also limits to the power of the Royal Prerogative, and those limits have been tested during the course of the Brexit process.
Their use to activate Article 50 was challenged by Gina Miller at the beginning of 2017.
The Supreme Court ruled in her case that the government could not trigger the EU exit process without bringing it before Parliament.
Because an act of Parliament had been required to trigger Article 50, the ECJ advocate general said: “It is logical, in my view, that the revocation of that notification also requires parliamentary approval.”
This does not mean it is a completely settled matter though.
As independent research group The UK in a Changing Europe points out: “Neither the advocate general nor the ECJ has the power to rule on the UK’s constitutional arrangements.”