Rebel Tory MPs, led by the former attorney general Dominic Grieve, may on Wednesday inflict the first defeat on the Government in votes on its important European Union (Withdrawal) Bill.
They are refusing to drop their amendment which insists that the UK should not leave the EU without "prior enactment of a statute by Parliament approving the final terms of withdrawal of the United Kingdom from the European Union".
Or to put it another way, they are insisting that it is written into law that MPs and Lords have the final say on the terms of Brexit.
As I write, ministers and whips are scrabbling around to see if the rebels can be bought off.
They have told them that there will be a written ministerial statement on Wednesday morning paraphrasing the rebels' demands as an alternative to the amendment.
However the truculent Tory MPs tell me that they will not withdraw the amendment.
One said that they had informed ministers that they would only back down if the Government itself presented its own equivalent amendment, but ministers have refused to take them up on their offer.
"I have told them to accept my amendment and if it in turn needs any amendment to do it at Report (or a later phase of the Bill's passage through parliament)", said an estranged Tory MP.
The Tory MPs expected to vote for the amendment, and against the instructions of the Prime Minister, include Mr Grieve, Ken Clarke, Nicky Morgan, Anna Soubry, Antoinette Sandbach, Stephen Hammond, Sarah Wollaston, and Heidi Allen.
One Government official conceded that the defeat would not be a disaster for the Prime Minister, unless it encouraged rebels in the Commons and Lords to make further and more damaging changes to the bill.
"We never like to lose, it would be embarrassing, but we would cope," he said.
Another said that the amendment might stop the Government being able to guarantee to EU negotiators that it could deliver on all its Brexit promises - such as to set up a new body by statutory instrument to monitor the rights of EU migrants living in the UK.
One rebel Tory MP said that this putative practical defect with their amendment was "laughable and absurd".