European Court of Justice judges has ruled the UK can unilaterally revoke Brexit without permission of the 27 member states.
The ECJ ruling comes ahead of Tuesday's crunch vote on Theresa May's controversial EU withdrawal deal which could see her ousted as Prime Minster.
The case to decide whether an EU member state such as the UK can decide on its own to revoke the Article 50 withdrawal process was brought by a cross-party group of Scottish politicians.
They are Labour MEPs Catherine Stihler and David Martin, Joanna Cherry MP and Alyn Smith MEP of the SNP, and Green MSPs Andy Wightman and Ross Greer, together with lawyer Jolyon Maugham QC, director of the Good Law Project.
They argued unilateral revocation is possible and believe it could pave the way for an alternative option to Brexit, such as a People's Vote to enable remaining in the EU.
“[The UK] is free to revoke unilaterally that notification”
Legal representatives for the UK Government argued the case is inadmissible as it deals with a hypothetical situation, since the Government's policy is not to revoke Article 50.
Lawyers representing the Council of the European Union and from the European Commission meanwhile argued that revocation is possible but would require unanimous agreement from all member states.
The case was originally heard by the Court of Session in Edinburgh and two attempts by the UK Government to appeal against the referral to the European court were rejected.
It will now be referred back to the Court of Session, where judges are expected to "frank" the decision and declare the European Court's answer to be the law on the matter.
A spokeswoman for the court said: "In today's judgment, the Full Court has ruled that, when a Member State has notified the European Council of its intention to withdraw from the European Union, as the UK has done, that Member State is free to revoke unilaterally that notification.
"That possibility exists for as long as a withdrawal agreement concluded between the EU and that Member State has not entered into force or, if no such agreement has been concluded, for as long as the two-year period from the date of the notification of the intention to withdraw from the EU, and any possible extension, has not expired."
Brexit will face a crunch vote in the House of Commons as the Prime Minister takes her deal between Britain and the EU before MPs.
Mrs May's plan – the Brexit Withdrawal Agreement – has been heavily criticised by Labour and her own MPs, and is thought to either make of break her future as PM.
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Monday's ruling upheld a finding by ECJ advocate general Manuel Campos Sanchez-Bordona, who said last week that Article 50 of the Lisbon Treaty allows the "unilateral revocation of the notification of the intention to withdraw from the EU, until such time as the Withdrawal Agreement is formally concluded".
He rejected the contention that the mechanism for a member state to quit the trade bloc could only be reversed following a unanimous decision of the European Council.
In Monday's ruling the judges ruled that a country's decision to revoke Article 50 after notifying the EU of its decision to withdraw "reflects a sovereign decision to retain its status as a Member State of the European Union, a status which is neither suspended nor altered by that notification", the spokeswoman added.
Mrs May's political future appears uncertain if she loses tomorrow's vote – but her credibility would also suffer a serious blow if she attempts to postpone the Commons vote.
Downing Street insisted the vote would go ahead as planned on Tuesday and Brexit Secretary Stephen Barclay said it was "100% happening".
But rumours continued to swirl at Westminster that Mrs May could decide at the 11th hour to avoid a potentially heavy defeat if Tory whips cannot limit the revolt over her Brexit deal and the controversial Irish backstop arrangement.
Debate on the plan will resume in the Commons on Monday, and Mrs May was warned not to give MPs the impression she lacks "appetite for the fight".
Calls to European Council president Donald Tusk and Taoiseach Leo Varadkar on Sunday prompted speculation that efforts to find a way of reassuring backbenchers over the backstop – contingency measures aimed at preventing a hard border on the island of Ireland – were ongoing.
Splits in the Tory ranks appeared even deeper after three former ministers publicly failed to rule out leadership bids and serving members of Mrs May's Cabinet appeared to be jockeying for position to replace her.
Former foreign secretary Boris Johnson pointedly refused to rule out challenging Mrs May, although he insisted it was "nonsense" to suggest he was already lining up members of his cabinet.
Asked to give an "absolute, categorical promise" that he would not stand against the Prime Minister, Mr Johnson told the BBC's Andrew Marr Show: "I will give you an absolute, categorical promise that I will continue to advocate what I think is the most sensible plan."
Ex-work and pensions secretary Esther McVey said she would seriously consider a run for the leadership if asked by colleagues to stand, while former Brexit secretary Dominic Raab refused to rule out his own bid for the top job.
The sense of crisis has heightened expectations that the 48 letters required to trigger a confidence vote could be submitted to the Tory backbench 1922 Committee this week – following a failed coup staged by Brexit cheerleader Jacob Rees-Mogg last month.