The European Court of Justice (ECJ) has ruled that the UK can cancel Brexit without the permission of the other 27 EU members.
The judges ruled that this could be done without altering the terms of Britain's membership.
A group of anti-Brexit politicians and campaigners argued that the UK should be able to unilaterally halt the Brexit process if it wants to do so.
But their case was opposed by the UK government and the EU itself.
A senior ECJ official - the advocate general - said last week he agreed the UK should be able to change its mind about leaving. His opinion was not legally binding, but the court tends to follow his advice in the majority of cases.
The decision comes the day before the House of Commons is due to vote on the prime minister's Brexit deal with the EU - with MPs widely expected to reject the proposals, says a BBC report.
The campaigners hope the victory in their legal case will increase the chances of Brexit being called off completely, potentially through another referendum.
This is because they believe it sends the message that MPs are not facing a choice between accepting May's deal or leaving the EU with no deal, and that "there are other options, and we can stop the clock."
The case was brought by a cross-party group of politicians, including Scottish Green MSPs Andy Wightman and Ross Greer, SNP MEP Alyn Smith and MP Joanna Cherry and Labour MEPs David Martin and Catherine Stihler, who have been supported by Jolyon Maugham QC of the Good Law Project.
They received a major boost last week when one of the ECJ's advocate generals, Manuel Campos Sanchez-Bordona, issued a statement which backed their position.
Campos Sanchez-Bordona said it was his opinion that if a country decided to leave the EU, it should also have the power to change its mind during the two-year exit process specified in Article 50 of the EU treaty.
Crucially, he also said it should be able to do so without the consent of the other EU states.
But he said the possibility of revoking the decision would be subject to "certain conditions and limits". This includes the withdrawal being a parliamentary decision, and observing "the principles of good faith and sincere co-operation... to prevent abuse of the procedure".
Dangerous precedent
This was despite the EU warning during a previous court hearing that it would set a dangerous precedent by encouraging other countries to announce they are leaving in an attempt to secure better membership terms, before cancelling their withdrawal.
The UK government's lawyers argued that the case was purely hypothetical as "the UK does not intend to revoke its notification", and ECJ judges should therefore have refused to rule on it.
They also claimed that the politicians behind it wanted to use the case as "political ammunition to be used in, and to pressure, the UK Parliament".
The case was initially taken to the Court of Session in Edinburgh, which ultimately agreed to pass it to the ECJ.
Two attempts by the UK government to appeal against the referral to the European court were rejected, and the case was heard by all 27 ECJ judges last month.