Rishi Sunak faces a reckoning from hardline Brexiters on his backbenches who are prepared to follow the Democratic Unionist party in voting against a key element of his Windsor framework that overhauls the Northern Ireland protocol.
Anger is rising among those in the European Research Group at ministers using a vote on the Stormont brake to implement the Stormont brake on Wednesday, as a proxy for MPs to have their say on the whole deal.
After weeks of examining the details of the agreement, which was sealed by Sunak and the EU Commission President Ursula von der Leyen on 27 February, the DUP signalled their discontent more strongly.
Jeffrey Donaldson, the DUP leader, said he was in Washington DC a few days ago: "It is my current assessment that there are key areas of concern that need further clarification, reworking and change as well as further legal text." Concerns have grown among Conservative whips that some Tory MPs could vote against or abstain on Wednesday, even though Labour s support guarantees the motion will pass.
The EU law will be subject to the vote on a statutory instrument that will implement a mechanism designed to appease concerns about the jurisdiction of the EU.
The UK government can veto any new EU laws that apply to trade in Northern Ireland under the new Windsor framework.
Several Tory MPs privately suggested that they might follow suit after a senior DUP source told the Sunday Telegraph the party was likely to vote against the government on Wednesday.
Peter Bone, the former deputy leader of the Commons, said he was very unhappy about the statutory instrument vote, which was treated as a chance for MPs to have a say on the Windsor framework.
He said that he has not been given a reasonable explanation as to why it is being done that way.
The government is rubbing me up the wrong way when it comes to this. The whole thing about the Windsor framework is that everyone agrees that it is an improvement, but this isn't going to be a temporary measure, this is permanent.
It is a bit like a budget. When it was announced, it all sounds good but then you have to go away and look at the small print to see how good it really is. I haven't made my mind up yet. The ERG is expected to hold a press conference on Tuesday to announce the verdict of its star chamber of legal advisers, who have been looking at the details and implications of the Windsor framework.
Oliver Dowden, the Cabinet Office minister, said he was confident that the vote would pass, as he said that he was confident that the DUP would come on board. Sunak wouldn't need to be in hock to his party in order to pass a deal. Lisa Nandy, the shadow levelling up secretary, told Sky: There is no doubt that this is something that is urgent. It is incredibly important, and trying to remove some of the friction on the island of Ireland has been our priority for a long time.