Downing Street has warned Tory backbenchers and Northern Ireland unionists that they will go ahead with their Brexit deal with or without their support.
A Number 10 source said Rishi Sunak was confident the measure was going to get through Parliament as it “removes the threat of any ratchet of EU law”.
MPs will be given the chance to vote on the Windsor Framework, Mr Sunak’s agreement with the EU to solve post-Brexit problems in Northern Ireland, on Wednesday.
The vote will concern the so-called Stormont brake, which the Government says allows Britain to oppose the imposition of new EU law on Northern Ireland.
One source said the Democratic Unionist Party was “hardening” against the agreement although the party is continuing to demand clarifications from Downing Street about aspects of the agreement.
The European Research Group of pro-Brexit Tory MPs is also considering the plan. One of its senior members, Sir Bill Cash, has been analysing it for weeks and he will present his findings soon.
However, it is understood he has concluded that the deal still gives too much power to the EU as it will make it hard for the UK to diverge from Brussels regulations.
A Number 10 spokesman said: “We’re confident that this is the best deal for Northern Ireland, which meets the tests we set, and we are confident that MPs will support it, including by voting for the Stormont Brake.
“The Stormont Brake is the most significant part of the agreement, putting power back into the hands of Stormont and Westminster, safeguarding Northern Ireland’s sovereignty and removing the threat of any ratchet of EU law and the ECJ.
“This vote enables Parliament to have its say.”
The source added: “The Framework has been overwhelmingly welcomed and we continue to engage with all of our backbenchers and answer any questions they may have.
“We aren’t working to the timetable of one group or individual, but we have been extensively engaging with the DUP to answer their questions and continue to do so.”
How Boris Johnson decides to vote will be watched closely after he said earlier this month he would find it “very difficult” to support the agreement in the Commons, adding: “I believe we should have done something different, no matter how much plaster came off the ceiling in Brussels.”
However, the former prime minister conceded Mr Sunak had the “momentum” over the new agreement, which most Tory MPs are expected to back. Labour has also confirmed it will back the deal.
Conor Burns, the former Northern Ireland minister and an arch-Brexiteer, could not say whether the DUP would come on board with the agreement but struck a cautiously optimistic note.
“On balance, it’s probably about as good as we’re going to get,” Mr Burns told the Sunday Times. “I think Rishi has done well… Immodestly, I think what we did last summer [the Northern Ireland Protocol Bill] accelerated it.”
It came as Miguel Berger, Germany’s ambassador to the UK, praised Mr Sunak for developing a “relationship of trust” with the European Union after a “very low point [of] no trust” during Mr Johnson’s time in office.
“I think it’s a very good compromise that has been worked out quietly over four months between the European Commission and the British Government,” Ms Berger said in an interview with Sky’s Sophy Ridge.