On the second day of debate on changes to May's EU withdrawal bill, lawmakers will vote on amendments handed down from the upper house of parliament over Britain's relationship to the bloc's customs union and single market.
And it is Labour's leadership which is on the alert for rebellion among its MPs, after ordering them not to back proposals for a Norway-style agreement with the European Union.
A dispute quickly arose about the third clause of Grieve's amendment, with Brexiters, including some ministers, quickly saying all that had been agreed was talks - and no government could agree to be "directed" by MPs.
In a day of drama, May's position seemed suddenly weaker when junior justice minister Phillip Lee, who has always been critical of Brexit strategy, resigned and said he would vote against the government.
Remainer Stephen Hammond told BBC Radio 4's PM programme: "We have spoken in a room with the Prime Minister this afternoon, 10 minutes before the first round of voting".
"It would tie the hands of the Government in a way that I think could make no deal more likely", said Mr Buckland.
A Department for Exiting the European Union spokesman said: "The Brexit Secretary has set out three tests that any new amendment has to meet - not undermining the negotiations, not changing the constitutional role of Parliament and Government in negotiating global treaties, and respecting the referendum result".
But Brexit campaigners feared it could weaken Britain's negotiating stance in talks to leave the European Union and the Brexit ministry was quick to put out a statement saying: "We have not, and will not, agree to the House of Commons binding the government's hands in the negotiation".
A series of further votes will take place on Wednesday, with no defeats expected after ministers agreed compromise wording over post-Brexit plans for a "customs arrangement".
Dominic Grieve withdrew his own amendment, which would have given MPs powers to dictate what the Government should do if no acceptable agreement is reached by February 2019. If the government fails to pass the bill as it is, it will be forced to change what it asks for in negotiations with the European Union -undermining May's position and possibly threatening her job as Prime Minister.
Believe me, in Westminster, there is a very very big difference between the promise of more serious chat about something with the possibility of a change - and a promise actually to do something different, especially if it is made by the occupant of Number 10.
The government said it would produce its revised plan on Thursday.
Ministers sought to play down the concessions.
She told LBC that she doesn't want to stop Brexit, but is determined to ensure she can get the best deal for her constituents. Now the focus shifts to the price of the rebels' compliance, and it could be a high one for the beleaguered prime minister. "It was the prime minister who I sat in front of this afternoon and who gave us those assurances". "I am confident I can get a deal that allows us to strike our own trade deals while having a border with the European Union which is as frictionless as possible".