Theresa May has stepped up last-ditch efforts to try to win over Brexit-backing MPs after government legal advice warned the Irish backstop could leave the UK trapped in “protracted and repeated rounds of negotiations” for years to come.

But Brexiters immediately rejected one idea mooted by Downing Street, of promising a “parliamentary lock” – giving MPs a vote before the backstop could be implemented.

The prime minister is holding a series of face-to-face meetings with groups of MPs, seeking to persuade them there is no viable alternative to her approach.

With just six days to go until the vote on her controversial deal, which May is expected to lose heavily, Downing Street confirmed the prime minister was keen to find ways to offer MPs extra reassurance about the backstop, in the hope they will support her.

But Steve Baker, of the European Research Group (ERG), dismissed the parliamentary lock plan as “silly”; while Jacob Rees-Mogg said it would require the 585-page withdrawal agreement to be renegotiated – something No 10 has insisted is impossible.

Another senior Brexiter called the idea a “risible, transparent ruse”; and dismissed the No 10 strategists who came up with the compromise as “twerps”.

The Conservative chief whip Julian Smith attended a private ERG meeting of about two dozen Tory MPs on Wednesday night, which was also attended by the DUP.

Sources on both sides said that nothing new had been tabled by Downing Street and that the whip was in “listening mode”. He “answered lots and lots of questions”, said a person present, “but it wasn’t about doing deals, it was about listening and reporting back”.

Separately, Rees-Mogg and Baker held a meeting with Nigel Dodds, the Westminster leader of May’s DUP informal coalition partners.

An ERG statement said Dodd told them the DUP is so set against May’s Brexit plan that if it passed the Commons, the party would rescind its pledge to back her in any vote of confidence, placing the government in peril. If her deal falls they would support her.

The Times reported [paywall] that some cabinet members were even pushing for a delay to the Brexit vote amid fears that the defeat would be so catastrophic it could bring down the government. However, a Downing Street spokesman dismissed the suggestion. “I don’t doubt a couple of people think [it should be delayed] but that’s different to saying that’s what’s going to happen,” he said.

Amid a febrile atmosphere in Westminster ahead of Tuesday’s vote, the Guardian also learned the Unite general secretary, Len McCluskey, had privately told Labour MPs the party should have severe reservations about backing a fresh referendum, saying voters could see it as a betrayal.

Labour’s deputy leader, Tom Watson, hit back at McCluskey’s warning, saying that to suggest a second referendum would represent a betrayal was a “gross distortion” of the party’s position.

At the heart of the Tory row over whether MPs should accept the prime minister’s deal is the legal status of the arrangements for preventing a hard border between Northern Ireland and Ireland – and, in particular, the UK’s ability to extricate itself from this.

Fresh details emerged on Wednesday as the attorney general, Geoffrey Cox, finally published his full legal advice about the deal. The six-page document, a letter from Cox to May, was released after MPs voted the day before to hold the government in contempt of parliament for its repeated refusal to do so. The advice confirmed that, as Cox conceded in his Commons statement on Monday, the UK could remain “indefinitely” in the backstop.

He wrote that the arrangements for avoiding a hard border could “endure”, even if negotiations between the two sides broke down. Cox also warned that disputes about whether the UK should be able to exit the backstop could become intractable.

“In the absence of a right of termination, there is a legal risk that the United Kingdom might become subject to protracted and repeating rounds of negotiations,” he concluded.

“This risk must be weighed against the political and economic imperative on both sides to reach an agreement that constitutes a politically stable and permanent basis for their future relationship. This is a political decision for the government.”

Cox’s advice, which hardened the resolve of some Brexiters to vote against May’s deal, was published as the debate about the deal in the House of Commons entered the second of five scheduled days. As with the inaugural day, there was scant evidence of MPs on either side being convinced to change their minds.

A string of former cabinet ministers, including Justine Greening and Michael Fallon, outlined what they saw as the deal’s failings, with Greening calling for a second referendum and Fallon saying he could only accept the deal with four separate changes.

The theme for the second day was security and immigration, which meant the home secretary, Sajid Javid, opened proceedings, telling MPs he could not “think of a better way to celebrate my 49th birthday”. While noting that he could not insist the deal “is perfect in every sense”, Javid urged MPs to back it, warning of potentially lasting damage to security and policing if rejecting the plan resulted in a no-deal departure.

But Sam Gyimah, who resigned as a junior education minister on Friday, joined Fallon and Greening in explaining why he could not support the government.

“The home secretary admitted it might not be perfect, almost implying this is like trying on a pair of shoes, not the right colour, maybe a little bit tight but you could get on with it and life would be fine,” he said.

“But actually this is like shoes that have holes in the soles. This deal is fatally flawed.”