Tory Brexiteers, dubbed “the Spartans”, are reportedly split on whether to support Prime Minister Boris Johnson’s proposed exit treaty.

Nigel Farage and the Democratic Unionist Party (DUP) have warned that the new EU-approved Brexit deal fails to deliver on the June 2016 referendum vote and threatens the Union.

While the 28 Spartans in the European Research Group (ERG) were expected to follow the DUP in not backing the deal, The Guardian reports several are considering voting for it.

One source said there were concerns raised at last night’s ERG meeting over continued the superiority of the European Court of Justice and the DUP losing its veto.

“There were a lot of mixed views. This vote will cause some agony for some members, but it will be up to each member’s conscience,” they said.

Another said: “We have stood with the DUP for so long that for some members, it will be painful to choose.”

The newspaper reports that Peter Bone and Andrew Bridgen had suggested they were going to vote for it and Owen Paterson had not ruled it out, either. ERG chief Steve Baker and his deputy Mark Francois were said to have made ‘positive noises’ about it during the meeting and that they only had “limited remaining concerns”.

Labour MPs from Leave-backing constituencies may also feel compelled to back the deal. Jim Fitzpatrick, the Labour MP for Limehouse and Poplar, said he would vote for the deal, adding that up to 20 Labour MPs could back it, as well.

ITV’s political editor Robert Peston believes there is “genuinely a chance” the bill could pass tomorrow. “I never ever said that about any of Theresa May’s Meaningful Votes,” he added.

Mr Peston believes that 18 or 19 of the Tory Rebels — Remainers and soft-Brexiters who were expelled from the party for voting against the government — could back the deal. He believes that whilst the DUP remains a “challenge”, “the ERG Spartans will fear if they vote against the deal and the WAB, that Brexit delay and no Brexit will follow”. Earlier this week, leader of the House of Commons Jacob Rees-Mogg said that “the votes are now there” for Mr Johnson to pass an exit deal through parliament.

Former Prime Minister David Cameron — who legislated the referendum, campaigned for Remain, and resigned the day after citizens delivered a Leave result — said he would vote for the deal if he were still an MP. Comparing the current prime minister to a “greased piglet”, he said he believed Mr Johnson’s bill would pass on Saturday — but only just.

“The thing about the greased piglet is that he manages to slip through other people’s hands where mere mortals fail,” he said during a talk to promote his book For the Record.

“The country voted to leave the European Union, the best way to leave is with a deal, I think a no-deal Brexit would be bad for the economy and bad for the union.

“I think it’s much better to leave with a deal, and I think Boris has done well to achieve that deal. I hope he’ll get it through parliament, I suspect he will but it will be tight,” Mr Cameron said in comments reported by The Guardian.