The European Court of Justice has ruled the UK can cancel Brexit without the permission of the other 27 EU members.
The ECJ judges ruled this could be done without altering the terms of Britain’s membership.
A group of anti-Brexit politicians argued the UK should be able to unilaterally halt Brexit, but they were opposed by the government and EU.
The decision comes a day before MPs are due to vote on Theresa May’s deal for leaving the EU.
MPs are already widely expected to reject the proposals during a vote in the House of Commons planned for Tuesday night.
But there is now speculation the vote will be cancelled, with three sources confirming the delay to the BBC’s political editor Laura Kuenssberg.
Mrs May will give a statement to the House at 15:30 GMT, and Brexit Secretary Steve Barclay will give a statement responding to the ruling after that.
What was the court case about?
The case was brought by a cross-party group of Scottish politicians and the Good Law Project who wanted to know whether the UK could revoke the decision to leave the EU without getting approval from the other member states.
They believed that if the ruling went in their favour, it could pave the way for an alternative option to Brexit, such as another referendum.
Both the UK government and the EU had been against it going to the ECJ.
The EU warned that it would set a dangerous precedent by encouraging other countries to announce they were leaving in an attempt to secure better membership terms, before cancelling their withdrawal.
The UK government’s lawyers also argued that the case was purely hypothetical as “the UK does not intend to revoke its notification” and those politicians behind it wanted to use the case as “political ammunition to be used in, and to pressure, the UK Parliament”.
What does the ruling say?
The ECJ ruled that the UK can unilaterally revoke its withdrawal from the EU, broadly following the non-binding opinion given last week by a senior ECJ official – the advocate general.
The statement from the court said the ability for a member state to change its mind after telling the EU it wanted to leave would last as long as a withdrawal agreement had not been entered into, or for the two-year period after it had notified the bloc it was leaving.
If that two-year period gets extended, then a member state could change its mind during that extra time too.