Theresa May has insisted MPs will be given a vote on the Brexit withdrawal deal well before the UK leaves the EU as she faces her first Commons defeat on the issue.
The prime minister and Brexit secretary David Davis are seeking to fend-off a Tory rebellion led by former attorney general Dominic Grieve ahead of a crunch vote on the European Union (Withdrawal) Bill.
Mr Grieve says he has enough support to defeat the government on Wednesday evening unless the Bill is changed to require any final Brexit deal to be approved by a statute before it is implemented.
The statute sought by Mr Grieve would undergo full parliamentary scrutiny - meaning it could be rewritten by MPs. This could potentially leave the government vulnerable to further revolts over elements of the withdrawal deal.
Mr Grieve told the Commons he would not back down on his commitment to the amendment: "There's a time for everybody to stand up and be counted, as Churchill said, he's a good party man, he puts the party before himself and the country before his party.
"And that's what I intend to do."
At Prime Minister's Questions, Mrs May said: "We will put the final withdrawal agreement between the UK and the EU to a vote in both Houses of Parliament before it comes into force."
Under pressure from pro-EU Tory Anna Soubry to accept the former attorney general's amendment, Mrs May repeated that there would be a "meaningful vote" in the Commons, followed by the legislation in the Withdrawal Agreement and Implementation Bill.
Westminster would be given a vote ahead of the European Parliament and "well before" the date of Brexit in March 2019, she said.
"To be clear, the final deal will be agreed before we leave and Honourable and Right Honourable Members will get a vote on it," she told the Commons.
Mrs May said the withdrawal agreement with Brussels would be put into legislation "which will be subject to full parliamentary scrutiny".
But she said Mr Grieve's proposal as currently drafted could mean the UK is not able to have the "orderly and smooth exit from the European Union that we wish to have".
Mr Davis wrote to Tory MPs and issued a written ministerial statement committing the government to "a number of votes" on the final deal struck with Brussels.
Labour is set to back Mr Grieve and urged would-be rebels not to be bought off by "warm words and woolly concessions".
The prime minister's lack of a majority leaves her vulnerable to any Commons revolt and, with up to 20 Tory MPs set to side with Mr Grieve in the division lobbies, she could face a defeat.
Mr Grieve said it would be the second time he has rebelled in his 20-and-a-half years as an MP, as he seeks to amend the European Union (Withdrawal) Bill.
Speaking during day seven of the Bill's committee stage, Mr Grieve said: "The purpose of the amendment, the nature of it is entirely lost in a confrontation in which it is suggested that the underlying purpose is the sabotage of the will of the people, which it most manifestly is not.
"That's then followed by the hurling of public abuse, large numbers of people telling one is a traitor."
Mr Grieve said last week he thought he was on the verge of an agreement with ministers over the powers but then "everything closed down", adding there is still a need for them to provide MPs with "chapter and verse" as to why they are needed.
He said: "So in those circumstances the only proper course of action - and I say it with the greatest reluctance - is that I'm going to have to vote against the government, or vote for my amendment - and if necessary if it's not passed I will vote against clause nine.
"Because without my amendment clause nine becomes a really very worrying tool of executive power which doesn't appear to have any reasonable presence in this legislation."
The prime minister's official spokesman said: "We have, in good faith, come forward with a strong package of concessions to deal with the spirit of the amendment."