Boris Johnson has vowed to oppose Rishi Sunak's new Brexit deal for Northern Ireland in a crunch Commons vote later today.
The ex-Prime Minister will rebel against Mr Sunak's Windsor Framework - which overhaul's Mr Johnson's old Brexit deal governing Northern Ireland after years of deadlock.
Mr Johnson said the plan was "unacceptable" and argued that his own law-flouting Northern Ireland Protocol Bill was the best way to proceed.
Northern Ireland Minister Steve Baker said Boris Johnson risked “looking like a pound shop Nigel Farage” by voting against part of Rishi Sunak’s Brexit deal.
Liz Truss has also decided to rebel against her successor's plan, a source close to the former Prime Minister said.
She believes the pact fails to "satisfactorily resolve the issues thrown up by" the Northern Ireland Protocol and "almost fatally impinges" on the UK's ability to diverge from EU rules and regulations, the source said.
Ex-Home Secretary Priti Patel will vote against the deal, telling the Telegraph: "I will not be buying shares on the Government's smoke and mirrors on Windsor."
MPs will be asked to vote on the Stormont Brake, a key part of Mr Sunak's Brexit plan today - which is being seen by No10 as a verdict on the whole deal.
In dramatic scenes, Mr Johnson could have to pause his evidence about whether he lied to MPs about Partygate to go and vote.
He said: "The proposed arrangements would mean either that Northern Ireland remained captured by the EU legal order - and was increasingly divergent from the rest of the UK - or they would mean that the whole of the UK was unable properly to diverge and take advantage of Brexit.
"That is not acceptable. I will be voting against the proposed arrangements today.
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"Instead, the best course of action is to proceed with the Northern Ireland Protocol Bill, and make sure that we take back control."
The vote is expected to pass with Labour support but Mr Sunak was dealt a blow when a group of hardline Tory Brexiteers branded it "practically useless".
The European Research Group (ERG) did not reveal how they would vote but their highly critical report - and Mr Johnson's decision - suggests that they will rebel.
The Democratic Unionist Party have all said their 8 MPs will oppose it.
Tory backbencher Peter Bone said he was "pretty miffed" about the Government's approach to a vote - and suggested he could also rebel.
"We were promised a full debate on the Windsor Framework," he told Sky News.
"If I get a question at PMQs I'm going to ask the Prime Minister what happened to our wider vote?
"So I'm really pretty miffed that the Government is avoiding scrutiny on this and on the brake itself it seems to fail all the tests.
"If that is the case, I'm going to listen to the debate. I'm going to go meetings this morning, but if I had to vote at this moment in time, I should vote against."
Mr Baker said Mr Johnson - and Liz Truss who is also refusing to vote for it - were “both better than this” and should be backing the Windsor Framework today.
He said: “We've partly reached this point thanks to Liz Truss setting the process in train.
“And today's measures are better, of course, than the protocol that Boris Johnson put in place, a protocol which he spoke about and those things he said turned out not to be accurate.
“So you know, he's got a choice: he can be remembered for the great acts of statecraft that he achieved, or he can risk looking like a pound shop Nigel Farage and I hope he chooses to be remembered as a statesman.”
When asked about the embarrassing possibility that Mr Sunak might have to rely on Labour to get his deal through, Mr Baker said: “It’s no question of it going through on Labour votes. It’ll be going through because it's the right thing for Northern Ireland.”
