In an opinion issued before a judgement by the European Court of Justice, Advocate General Manuel Campos Sanchez-Bordona rejected the contention that Article 50 can only be revoked following a unanimous decision of the European Council, made up of the EU’s member states.
The Scottish Government also welcomed the announcement and said it gives the UK an opportunity to avoid a no-deal Brexit.
The case has been brought by a cross-party group of Scottish politicians led by Green MSP Andy Wightman, as well as UK legal experts.
A ruling is expected in the coming days and could be issues before the Commons votes on Theresa May’s Brexit deal on 11 December.
Lord John Kerr, a former UK ambassador to the EU and one of the architects of Article 50 who is taking part in the case, said: “This is no surprise and, if it the Court agrees, will confirm that it is still up to us to decide whether we want to keep the existing deal we’ve got in the EU rather than leave on the Government’s terms.
“The choice facing us is not simply between the Government’s deal and no deal. We can choose to change course.
“There is still time and, until the UK has left the EU, the Article 50 letter can be withdrawn.
“And despite some of the bogus claims that have been made by those who oppose staying in the EU, there would be no price to pay – political or financial – if we were to take back the Article 50 letter.
“With support growing across the country for a People’s Vote, it is clear to me that this is the best way forward. The people should have the right to choose.”
The Scottish Government’s Constitutional Relations Secretary Michael Russell welcomed the opinion and said: “Rejecting a bad deal must not mean leaving with no deal. It is a tribute to the perseverance of the Scottish parliamentarians involved that we will finally get clarity from the Court of Justice on this important question.”
Calling on MPs to seize the opportunity presented by the case, Mr Wightman said: “It is now highly likely that, if the people of the UK were to change their minds and decide to remain in the EU, there is now a route to doing so.
“This would involve the extension of the Article 50 notification period and a second referendum.
“This is the only option that ends the current chaos and provides a considered and sincere means by which the citizens of the UK can have the final say in this process.”
Catherine Stihler, the Labour MEP for Scotland and a member of the group involved in the legal process, said the Advocate General had “made clear that the UK can stop the ticking clock of Brexit before it is too late.”
Ms Stihler said: “If judges accept his opinion, the UK will have the option of halting the process, and will be able to offer the chance to keep the best deal we have as a member of the EU through a People’s Vote - rather than choosing between Theresa May’s bad deal or a catastrophic no-deal scenario.
“I pay tribute to Andy Wightman for leading on this important issue, which the UK Government had tried to stymie at every turn. There is now light at the end of the tunnel.”
Liberal Democrat Brexit Spokesperson Tom Brake said: “Liberal Democrats have long said that we can get out of this Brexit mess if we chose to.
“As MPs begin to debate Theresa May’s terrible deal, many will be reflecting on how it compares with the deal we’ve currently got.
“UltImately the Government must give a final say through a People’s Vote, with the option to remain in the EU.”
Jo Maugham QC of the Good Law Project, which is supporting the legal action, said the opinion “puts the decision about of future back in the hands of our own elected representatives - where it belongs.”
Mr Maugham added: “On this critical issue I’m sure MPs will now search their consciences and act in the best interests of the country.”