Anti-Brexit campaigners have been given permission to take their case to Europe’s highest court as they seek a ruling on whether it can be halted.
The cross-party group of politicians argue that Article 50 can be revoked if MPs vote to do so.
The Court of Session in Edinburgh had previously rejected their bid to have the case referred to European judges.
But they have now won an appeal, and the European Court of Justice will be asked to give a definitive ruling.
The panel of appeal judges at the Court of Session said the “urgency of the issue” – with the UK due to leave the EU on 29 March – meant its request to the European Court was being done under expedited procedure.
The UK government said it was “disappointed” by the decision and was giving it “careful consideration”.
But a spokesman stressed that the government remained committed to implementing the result of the EU referendum and “will not be revoking Article 50.”
The legal case has been brought by politicians including Scottish Green MSPs Andy Wightman and Ross Greer, Labour MEPs David Martin and Catherine Stihler and SNP MEP Alyn Smith, who have claimed that Brexit is “not inevitable” and “there is still time to change course”.
Welcoming the ruling, Mr Greer said: “If negotiations collapse, as appears to be happening, we have to know that a no deal disaster is not the only option on the table.”
The politicians have been joined by lawyer Jolyon Maugham QC, the director of the Good Law Project, who said the latest ruling was a “bombshell” that could “decide the fate of the nation” and potentially allow the country to “wake up from the nightmare that is this government’s Brexit”.
Prime Minister Theresa May has admitted that negotiations with the EU have reached an “impasse” after her Brexit plans were rejected at a summit in Salzburg earlier this week.
But in a speech outside Downing Street she insisted that: “Nobody wants a good deal more than me – but I will not overturn the result of the referendum, nor will I break up my country.”
The speech was described as “dreadful” by First Minister Nicola Sturgeon, who claimed Mrs May’s so-called Chequers proposals for Brexit were now a “dead duck” and that Brexit “should not happen” if the PM was not going to keep the country in the single market and customs union.
Unilaterally halt Brexit
The petitioners argue that the UK should now effectively be allowed to change its mind on Brexit, without needing the permission of the other 27 EU members.
If it is successful, their case could strengthen the hand of any attempt by MPs to keep the UK in the EU after the final details of its departure terms are known.
This is because it would give parliament the power to unilaterally halt Brexit if it feels any final deal – or no deal – is unacceptable, even if the government wants to leave regardless.
Court of Session judge Lord Boyd ruled in June that the case could not go to the European Court in Luxembourg as it was “hypothetical” and did not reflect political reality as it “seems highly unlikely that this government will revoke the notification”.
The campaigners appealed against that decision, and on Friday the court ruled in their favour.
The ruling was delivered by Scotland’s most senior judge, Lord Carloway, and his colleagues Lord Menzies and Lord Drummond Young.
The appeal judges said matters had “moved on” since Lord Boyd’s original ruling, with the European Union (Withdrawal) Act now setting out how parliamentary approval is to be sought once the negotiations between the UK government and the EU conclude.
Lord Carloway said it was therefore “clear” that MPs at Westminster would be required to vote on any Brexit deal agreed by the EU and the UK government.
‘Clarifying the options’
He stated: “It seems neither academic nor premature to ask whether it is legally competent to revoke the notification and thus to remain in the EU.
“The matter is uncertain in that it is the subject of a dispute; as this litigation perhaps demonstrates.
“The answer will have the effect of clarifying the options open to MPs in the lead up to what is now an inevitable vote.”
The judge also said the European court would not be advising parliament on “what it must or ought to do”.
Instead, he said it would be “merely declaring the law as part of its central function”, adding that “how parliament chooses to react to that declarator is entirely a matter for that institution”.
In their draft reference to the European Court, the judges ask: “Where a member state has notified the European Council of its intention to withdraw from the European Union, does EU law permit that notice to be revoked unilaterally by the notifying member state?
“And, if so, subject to what conditions and with what effect relative to the member state remaining within the EU?”