Theresa May is facing one of the greatest clashes of her premiership as MPs from across parliament contemplate accusing her of contempt of parliament over her refusal to publish legal advice she has received on Brexit.

MPs may move against the Prime Minister tomorrow, with figures from the DUP, Labour, Liberal Democrats and SNP planning to send a letter to Commons speaker John Bercow insisting the full advice be published.

Number 10 has been holding back detailed legal advice provided by attorney-general Geoffrey Cox on May’s withdrawal agreement. Last month, MPs passed a binding Commons vote which required the government to present any legal advice it had received “in full”, something May has ruled out several times.

Ministers have disregarded the motion, saying that Cox will tomorrow present a “full, reasoned position statement” on the advice – which critics, led by Labour shadow Brexit secretary Sir Keir Starmer, say would present an incomplete, politicised version of the advice.

The pressure was deepened today after the Sunday Times published details of the advice given to Cabinet, which included a passage saying the UK could end up being locked in a “backstop” arrangement with the European Union over the border with Ireland.

The paper reported on a letter sent by Cox, in which the top law officer said: “The [backstop] protocol would endure indefinitely” — and claimed Britain would only be able to lose the backstop through a trade deal, which could take years to reach and on which the EU would hold a veto.

Dominic Raab, who quit as Brexit secretary last month in protest against May’s deal, told the Sunday Times: “The legal position is clear. The backstop will last indefinitely, until it is superseded by the treaty setting out our future relationship, unless the EU allows us to exit.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. That’s my view as a former international lawyer, but it is consistent if not identical with all the formal advice I received.”

Over 100 Tory MPs are reportedly likely vote against the withdrawal agreement next week, according to lists compiled by Buzzfeed News and the Guardian. Combined with Labour, the Liberal Democrats and opposition from the DUP – which is in a confidence-and-supply agreement with May’s government – the bloc would likely be powerful enough to defeat the motion, and reports have suggested the government may even consider cancelling the vote entirely.

Speculation has grown about the concept of a “Plan B” – the path May will choose to take if her withdrawal agreement is voted down in parliament. The Prime Minister, who has been attending the G20 summit in Buenos Aires this weekend, has repeatedly declined to comment on suggestions she might try to call a general election.

Reports today say Starmer is prepared to sign a joint letter with the DUP’s Westminster leader, Nigel Dodds, the Liberal Democrat Brexit spokesman Tom Brake, and the SNP Europe spokesman Stephen Gethins, which will call for Bercow to allow a motion of contempt. If passed, the decision would pass to the committee of privileges, which would agree on a punishment if it decided contempt had occurred, which MPs would then need to approve. In the most extreme scenario, the committee could recommend expulsion.

Speaking to Sky’s Sophy Ridge this morning, Starmer said May was in “really deep water”.

“I don’t want to go down this path,” he said, adding: “with nine days to go until this vote, we shouldn’t be dealing with contempt of parliament.”

Writing in the Sunday Telegraph today, Starmer said: “MPs must have access to the fullest possible information about what they are voting on.”

“If the full advice is not forthcoming,” Starmer wrote, “we will have no alternative but to start proceedings for contempt of parliament – and we will work with other parties to take this forward… Although I accept the long-standing convention that Cabinet legal advice should be kept confidential, it is well-established that in exceptional circumstances that convention does not apply.”

Separately:

◙ A YouGov poll released today by the People’s Vote campaign, which is calling for a fresh referendum on the terms of Britain’s exit from the EU, found support for remaining in the bloc stood at 55 per cent – the highest result found by the polling group since the original vote in 2016. Universities and science minister Sam Gyimah, seen as a party loyalist, quit the government on Friday night over May’s handling of Brexit and suggested a second referendum may be needed.

◙ In Scotland, a cross-party motion set to be debated at Hollyrood next week is calling for the rejection of both May’s withdrawal agreement and a no-deal Brexit, saying a “better alternative” must be found.

◙ The clash between the leading parties over a televised debate on Britain’s withdrawal is continuing, with the Conservatives and Labour unable to settle on a format and time between several options presented by the BBC, ITV and Sky.