Theresa May is facing the "real possibility" of Commons defeat on her flagship Brexit bill unless MPs are guaranteed a "meaningful vote" on leaving the EU, Tory rebels have warned.
Former attorney general Dominic Grieve said he was determined to force a vote on an amendment to the EU (Withdrawal) Bill unless ministers back down.
With up to 20 Conservative MPs reportedly willing to support the change in the division lobbies on Wednesday, he said they could well have the votes needed to get it through.
"I think there are quite a few who may support me - I think enough, if this comes to a vote, to defeat the Government," he told BBC Radio 4's The World At One.
"I think there is a real possibility that that will happen."
The move threatened to wreck the fragile unity in the Conservative ranks in the wake of Mrs May's agreement in Brussels on Friday over the terms of Britain's withdrawal, potentially allowing the Brexit negotiations to move on to the second phase, including trade talks.
Brexiteers reacted angrily to the threatened revolt, accusing the rebels of trying to "derail" the whole bill, which is needed to transpose EU legislation on to the UK statute book when Britain leaves in March 2019.
Mr Grieve, who campaigned for Remain in the referendum, denied his amendment - which would require any final deal with the EU to be approved by a separate act of Parliament - was designed to prevent Brexit happening.
He said he tabled the change because, as the legislation stood, the Government had the power to do it by the use of statutory instruments, denying MPs the promised "meaningful vote" on the outcome of the negotiations.
He said that if ministers accepted the amendment, they could still come back at a later date with further changes to make it more to their liking.
"To keep this power in in the form which it is at the moment is plainly wrong," he said.
"I don't see any possibility of my backing down on this at all. I will vote for my amendment. I don't know whether I will be successful or not but I think I must push it forward.
"I don't want to defeat the Government. It is not my desire to do this, I would much prefer the Government to listen to what we have been saying, accept the amendment, and, at a later stage, the Government can always correct it if it wants to."
Ministers are expected to argue the amendment is unnecessary as they are already committed to tabling a separate bill to implement the main elements of any deal - including the agreements on the financial settlement and future citizens' rights signed by Mrs May on Friday.
Conservative former leader Iain Duncan Smith accused Mr Grieve and his supporters of "grandstanding" and trying to tie the Government's hands in the Brexit talks.
"I think this is looking for ways to derail the bill," he told The World At One.
"There comes a moment when really grandstanding has to stop. Tying the Government's hands in the way that he would wish to tie them so early on is quite wrong."
Asked about Mr Grieve's amendment, Mrs May's official spokesman said: "The Government position has been throughout that we are listening to MPs and we are having conversations with them, and where we think the legislation can be improved, we are prepared to take it on board.
"I think what the MPs are asking for is clarity. We are looking at the amendment and will respond to it in due course."