Boris Johnson and Liz Truss have both said they will vote against Rishi Sunak's Brexit deal.
Mr Johnson, who had already voiced concerns about the deal brokered with Brussels, confirmed he will not be backing the deal when MPs vote on the Stormont brake in the Commons later on Wednesday.
In a statement, the former prime minister 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.”
Meanwhile, Liz Truss was understood to believe Rishi Sunak's Windsor pact does not “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.
With Labour backing the Windsor Framework agreement signed last month, the government should win the Commons division comfortably, despite criticism from some hardline Tory Brexiteers.
The DUP has already said its eight MPs will vote against the regulation to implement the Stormont brake as it continues to seek changes to the overall framework.
The confirmation by Mr Johnson of his opposition to the UK-EU deal comes ahead of his appearance before the Privileges Committee, where he will be grilled by MPs investigating claims he knowingly misled Parliament over the “partygate” affair.
The former prime minister, who agreed the original Northern Ireland Protocol with Brussels as a way to avoid a hard border on the island of Ireland, had earlier this month indicated that he would find it “very difficult” to support the Windsor agreement.
On Tuesday the European Research Group (ERG) said the Stormont brake, which is intended to provide a veto on the imposition of new EU regulations in Northern Ireland, was “practically useless” following an analysis of the framework by its “star chamber” of lawyers.
Eurosceptic members have not yet decided how to vote, with the group set to meet later on Wednesday.
It comes as the former prime minister prepares to make a televised appearance before MPs investigating whether he knowingly misled parliament over Partygate.
In a statement ahead of Wednesday’s hearing of the privileges committee, the former prime minister said: “I look forward very much to the committee session tomorrow.
Mr Johnson said he accepts that he misled Parliament but said there was “no evidence” he did so intentionally, insisting his statements to the Commons regarding the Partygate scandal were in “good faith”.
He submitted his dossier of evidence to the privileges committee yesterday, while facing claims of bullying and intimidation as allies tried to discredit the probe.
In his legal argument, Mr Johnson insisted he was not warned that gatherings in Downing Street during the pandemic broke lockdown rules. He said that after learning they had, he corrected the record at the “earliest opportunity”.
