The Cabinet is on stand-by for fresh talks to agree a Brexit deal amid claims by Tory Leavers that the Prime Minister has already reached an agreement, while Mrs May will update European Council president Donald Tusk on the latest developments.
But 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.
Sir Jeffrey Donaldson, chief whip for Mrs May’s DUP allies, called for the advice to be published, as did shadow Brexit secretary Sir Keir Starmer.
The calls came as the Prime Minister set up five new panels to advise on adapting to the post-Brexit business world.
Leading figures including BT Group chairman Jan du Plessis and ITV boss Carolyn McCall will co-chair the telectoms, creative, technology and media business council.
Sir Roger Carr, of BAE Systems, and Rolls-Royce chairman Ian Davis will co-chair the industrial, infrastructure and manufacturing council and other senior figures will also chair business councils for the small business, financial and retail sectors.
Speculation that a Brexit deal is close was fuelled by leaked documents apparently setting out how the Prime Minister would sell an exit package to the public.
Downing Street distanced itself from the blueprint reported by the BBC but the document heightened suspicion among Tory Brexiteers that a deal has secretly been secured.
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.
Mr Gove wants to see the full legal advice setting out how an exit from the backstop can 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.
With Mrs May relying on the 10 DUP MPs for a Commons majority, Sir Jeffrey’s demand that the legal advice be published - something not normally done by governments - poses a fresh headache for the Prime Minister.
Sir Jeffrey told BBC Radio 4’s Today programme: “I think it’s in the public interest we understand fully what’s happening here.
“It’s because it affects the whole UK, therefore it shouldn’t just be the DUP that sees this advice, or the Government.
“If the House of Commons is going to have a meaningful vote on a deal that includes, and upon which this legal advice is very, very important, then I think people are entitled to know what that advice is.”
Labour’s Sir Keir, meanwhile, was due in Brussels to warn of the dangers of Britain and the European Union striking a loose agreement on future relations.
The shadow Brexit secretary told Today: “It is not the duty of the Opposition to back the Prime Minister come what may.
“What we can’t be expected to do now, with a gun to our head, is to back the Prime Minister whatever she brings back, however good or bad and without any detail.
“That is not opposition - that is surrender.”
He added that it was “essential MPs are given the opportunity to scrutinise the Attorney General’s legal advice before voting on the final deal”.
“The public have the right to know precisely what the Cabinet has signed up to and what the implications are for the future.”
Mrs May told senior ministers on Tuesday that 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 yet been scheduled ahead of the regular weekly gathering next Tuesday.
According to the leaked planning document, Mrs May will give a “we have delivered on the referendum” speech to the CBI conference on November 19.
The “grid” document said MPs would be told to put personal interests aside to back an agreement on November 27.
Former Brexit minister Steve Baker told the Daily Telegraph: “It looks like exactly the kind of back-of-the-envelope post-exit plan to sell a rubbish deal I would have expected.
“For months we have expected an orchestrated campaign following a fabricated row over the backstop.”
Jacob Rees-Mogg, chairman of the European Research Group of Tory Eurosceptics, told the newspaper: “There are rumours a deal has been done without keeping the Cabinet properly informed ... the Chequers proposals undermined trust.
“If there is an attempt to soften up and hoodwink people there will be no trust left.”
But Number 10 said the “childish language” and misspelling in the notes, which includes the incorrect spelling of the Irish prime minister’s name, should make it clear it “doesn’t represent the Government’s thinking”.