Boris Johnson and Liz Truss will both vote against Rishi Sunak’s Brexit deal in the Commons this afternoon.
Johnson said it was “not acceptable” that the Windsor Framework would still leave Northern Ireland subject to EU law.
He said the government should instead pass the Northern Ireland Protocol Bill, which would allow ministers to unilaterally rip up parts of the existing deal between the UK and Brussels.
Just two hours after Johnson’s announcement, it was confirmed that Truss - who defeated Sunak in last summer’s Tory leadership contest - will also join the growing Tory rebellion.
A source said she believes Sunak’s deal damages the UK’s ability to diverge from the EU’s rules and regulations.
The DUP have also said that they will vote against the deal.
Johnson 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. Instead, the best course of action is to proceed with the Northern Ireland Protocol Bill, and make sure that we take back control.”
Under the Windsor Framework, which Sunak agreed with European Commission president Ursula von der Leyen last month, the current customs border in the Irish Sea would be all-but removed.
Sunak has also claimed that a “Stormont brake” would allow the government to veto any new EU laws from being imposed on Northern Ireland.
However, the European Research Group of hardline Tory Brexiteer MPs yesterday said the mechanism was “practically useless” as they criticised the PM’s deal.
Despite the Tory and DUP opposition, the government is still likely to win this afternoon’s vote as Labour have already said they will support it.
However, a major backbench rebellion would be an embarrassment for the prime minister, and would store up problems for the future.
Today’s vote will take place while Johnson is being grilled by the Commons privileges committee over claims that he misled parliament over partygate.
It means he will have to break away from that appearance to vote against the government.