Theresa May is under intense pressure to publish the legal advice behind her Brexit plan as Labour, Tory Eurosceptics and the DUP line up against her.
Brexiteers, including Environment Secretary Michael Gove, want to see the full legal advice setting out how any customs arrangement to avoid a hard Irish border could be ended to avoid it becoming a permanent settlement.
The DUP, whose 10 MPs prop up the prime minister’s administration in the Commons, said it was “in the public interest” for the legal advice to be disclosed.
For Labour, shadow Brexit secretary Sir Keir Starmer said it was “essential” that MPs should be able to see the advice drawn up by Attorney General Geoffrey Cox.
Liberal Democrat Brexit spokesman Tom Brake said refusal to publish the advice “raises serious questions about what Tory ministers are trying to hide”.
In a development that will cause concern in Downing Street, a Tory European Research Group source indicated its MPs “would be up for sharing the attorney general’s wisdom” if Labour forced a Commons vote on the issue.
Pressure over the legal advice mounted as Cabinet ministers were invited to review the text of the withdrawal agreement which has so far been secured in negotiations with Brussels.
The prime minister told MPs last month that 95% of the deal had been agreed, although the key sticking point of the backstop to prevent a hard border between Ireland and Northern Ireland remained unresolved.
A Downing Street source said: “That is just where we are so far. It does not imply that a deal has been done.”
The development will do little to dampen speculation among Tory Eurosceptics that a deal is close to being signed off.
Brussels’ chief Brexit negotiator Michel Barnier said “more work is needed” but “we remain determined to reach a deal”.
Mrs May’s plan would see the whole UK effectively agree to remain in the customs union to help avoid a hard border with Ireland as a “backstop” if no other arrangement can be found.
The concerns focus on Mr Cox’s advice on how an exit from the backstop could be found to make sure the UK is not permanently kept within a customs union which would severely curtail its ability to strike free trade deals with nations around the world - a key prize for Brexiteers.
Legal advice is usually confidential, but DUP chief whip Sir Jeffrey Donaldson told BBC Radio 4’s Today programme that if the House of Commons was going to have a meaningful vote on the deal “then I think people are entitled to know what that advice is”.
Sir Keir, who visited Brussels for talks with senior EU figures, said: “At this crucial stage, Parliament should not be kept in the dark nor should the government try to bounce MPs into an agreement without all the facts.”
After a marathon session on Tuesday, the Cabinet is on stand-by for fresh talks to agree a Brexit deal if there are further developments.
Mrs May updated European Council president Donald Tusk on the situation on Wednesday morning.
He said the call was a chance to “take stock of progress” and discuss the way ahead.
Mrs May told senior ministers on Tuesday she would not agree a withdrawal deal with the EU “at any cost”.
The PM assured her top team that there would be another Cabinet before any agreement is settled.
No extra meeting has been scheduled ahead of the regular weekly gathering next Tuesday.