Secret legal advice which is believed to say that Brexit can be stopped unilaterally by Britain could be revealed, if a Commons vote is won next month.
Now two pro-EU MPs have tabled an amendment to the EU Withdrawal Bill which, if passed, would force the Prime Minister to publish a summary of that advice, they say.
The controversy could be crucial if MPs reject any Brexit agreement she secures in a “meaningful vote” to take place next year, before the withdrawal process can start.
Chris Leslie, a Labour MP and supporter of the pro-EU Open Britain group, said: “MPs and their constituents have every right to know the legal facts about what is and is not possible in the Brexit process.
“Ministers have provided no clarity on this so far and it’s about time they did.”
And Chuka Umunna, a fellow Labour MP who has co-tabled the amendment, added: “Ministers need to come clean about the legal advice they have received and give a simple answer to a simple question – can Article 50 be extended or withdrawn?
“People have every right to know that it can and that they still have a choice at the end of the Brexit negotiations as new facts emerge.”
Mrs May has insisted a rejection of her deal would simply mean Britain crashing out with no deal – insisting the Article 50 process is unstoppable.
But Lord Kerr, a former diplomat and the Article 50 author, argued the British Government could withdraw its letter and stop the process if it wished to.
Jean-Claude Piris, who served as legal counsel to the European Council for two decades until 2010, backed that interpretation, tweeting: “I am on same side as John Kerr.”
And a prominent QC said “two good sources” had told her that the Prime Minister has been advised that the exit notice can be reversed, if MPs judge that to be the best course.
Jessica Simor, from Matrix chambers, has called for the release of the legal advice under the Freedom of Information (FoI) Act. The Independent has made its own application.
Last month, Lord Callanan, a Brexit minister, was forced to apologise in the House of Lords for telling peers that Article 50 could not be revoked.
In October, Downing Street insisted it “never comments on government legal advice”, adding: “We have always been entirely clear that we are leaving the European Union.”
And Michel Barnier, the EU’s chief Brexit negotiator, argued the UK could not overturn the decision to leave without the consent of the other 27 EU member states.
Mr Leslie and Mr Umuna said they would table the amendment when the flagship EU Withdrawal Bill returns to the Commons for its report stage, on January 16.
The Bill completed its committee stage this week, but with continuing confusion over the status of the “meaningful vote” that MPs won, in the only defeat the Government suffered.
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