Opposition parties plan to join forces in a bid to force the government to publish the full legal advice it received ahead of the Brexit agreement.
“All parties” would press for contempt of Parliament proceedings if MPs are not shown the advice, Labour’s Brexit spokesman Sir Keir Starmer has said.
Theresa May has promised MPs only a “reasoned statement” on the legalities.
Many Brexiteers believe the Northern Ireland “backstop” provision offers the EU an effective veto on the UK leaving.
Mrs May insists the agreement’s legal text is clear that any backstop – keeping the UK under EU customs rules until a permanent trade deal was in place – would be temporary.
However, her former Brexit secretary Dominic Raab told the Sunday Times the backstop would last indefinitely – for as long as it takes to negotiate a new UK-EU relationship – “unless the EU allows us to exit”.
“The EU has a clear veto, even if the future negotiations stretch on for many years, or even if they break down and there is no realistic likelihood of us reaching agreement,” he is quoted as saying.
“That’s my view as a former international lawyer, but it is consistent if not identical with all the formal advice I received.”
The prime minister’s refusal to release the full advice prompted Northern Ireland’s Democratic Unionist Party – which has propped up Mrs May’s government since the general election in 2017 – to accuse her of having “something to hide”.
Ministers insist it is a long-standing convention that legal advice to the cabinet is kept confidential, and that government would otherwise be unable to function.
The row comes ahead of a crucial week for Mrs May, who faces opposition to her Brexit agreement from all sides of the Commons, with MPs due to vote on 11 December.
She spent much of last week trying to sell the deal to business people and the wider public, visiting Wales, Northern Ireland and Scotland.
And at the weekend she sought to reassure world leaders that the deal would be “good for the global economy”, when speaking at the G20 summit of the world’s most industrialised nations in Argentina.
On Saturday Mrs May lost science minister Sam Gyimah – the 10th resignation since she outlined her vision of Brexit – who warned the UK would be “hammered” in talks over a permanent EU trade deal.
She has also been unable to convince many of her own backbenchers of the deal’s merits, with Brexiteers in particular concerned about the effects of the backstop.
It was agreed as an “insurance policy” to prevent the return of customs posts along the Irish border, which many fear would reignite conflict.
And it involves the creation of a temporary customs territory, meaning the entire UK would continue to abide by EU rules in the event no trade deal was in place.
Last week, Mrs May told the Commons: “The legal text is clear that [the backstop] should be temporary, and that the Article 50 (withdrawal process) legal base cannot provide for a permanent relationship.
“There is also a termination clause which allows the backstop to be turned off when we’ve fulfilled our commitments on the Northern Ireland border. And there is a unilateral right to trigger a review… and the ability to seek independent arbitration if the EU does not use good faith in this process.”
Last month MPs approved a motion demanding full publication of the government’s legal advice. But on Monday Attorney General Geoffrey Cox, who wrote the advice, will offer only a limited summary of the government’s legal position during a statement to parliament.
Shadow Brexit secretary Sir Keir wrote in the Sunday Telegraph: “If the full legal advice is not forthcoming, we will have no alternative but to start proceedings for contempt of Parliament – and we will work with all parties to take this forward.
“The full legal implications of this deal clearly need to be known and debated in full by our Parliament.”
If contempt proceedings were requested, it would be up to Commons Speaker John Bercow to decide whether a debate and vote should be held.
BBC political correspondent Ben Wright said: “The argument threatens to further complicate a vital week for the government as the debate begins on the deal Theresa May is trying to sell to deeply sceptical MPs on all sides of the House.
“With ministers warning a defeat could derail Brexit altogether, today a separate cross-party group of MPs calls for parliament to support an amendment for another referendum at the earliest opportunity.”
Mrs May has repeatedly refused calls to put the issue of EU membership back to a public poll. Instead, she is focused on trying to win next week’s Commons vote.
Speaking in Buenos Aires on Saturday, Mrs May said: “The next nine days are a really important time for our country leading up to the vote on this deal.
“I will be talking with members of Parliament obviously and explaining to them why I believe this is a good deal for the UK.
“Why it is a deal that delivers on Brexit but it is also a deal that protects jobs and the economy and why passing this deal in the vote that takes place in the House of the Commons will take us to certainty for the future, and that failure to do that would only lead to uncertainty.”