The European Court of Justice will formally rule on whether Britain can unilaterally cancel Brexit ahead of the MPs vote on Theresa May's deal next week.
The EU's top court confirmed it will deliver its judgment at 8am UK time on Monday – with the landmark Commons vote scheduled for the next day.
Campaigners had asked the ECJ to rule on whether Article 50, which legally started the Brexit process, can be revoked by the country that triggered it. The text of the treaty clause is unclear and subject to dispute.
The ruling will come after the court's top legal advisor, the EU advocate general, recommended this week that the UK be given the power to revoke Article 50 without the consent of the EU's other member states.
If the recommendation is followed, as the advocate general's suggestions usually are, then an accidental no-deal Brexit would effectively be taken off the table, as the Government could stop it in any circumstances.
The UK Government's lawyers had argued that the case was hypothetical and should not be ruled on. The Government itself has been trying to use the threat of a no-deal to scare remain-leaning MPs into backing Theresa May's withdrawal agreement – a weapon this ruling would severely blunt.
Lawyers acting for the European Council and European Commission argued that the UK should not be able to unilaterally revoke the clause because it would allow wayward member states to cause trouble by invoking Article 50 with no intention of leaving.
But the advocate general said that as member states were sovereign countries they could still reverse commitments in international treaties before they were enacted.
The ruling on Monday will represent one of the fastest judgments ever issued by the ECJ. The process, which normally takes months or even years, has been sped up because of the time-sensitive and high-profile nature of the case.
On Tuesday MPs voted to put themselves in the driving seat in the PM's deal is voted down on Tuesday, amending legislation to ensure that they will get a vote on what to do next.
Government lawyers have already conceded that it is parliament rather than ministers that has the power to instruct a withdrawal of Article 50 – an assertion they made in a submission to the live case on revocation.
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