Theresa May is facing a potential backbench revolt, with rebel Tories threatening to vote with Labour and the SNP to change the Brexit Bill.
The EU (Notification Of Withdrawal) Bill returns to the Commons for a further three days of debate during committee stage, giving MPs the chance to amend the legislation.
Pro-Remain Conservatives are seeking assurances Parliament will get a say on the "endgame" if the Brexit negotiations collapse without a deal with the remaining 27 member states.
The Prime Minister has promised MPs and peers will be given a vote on any agreement she reaches in the talks with Brussels.
However pro-Remain MPs fear that if the negotiations fail, Britain would simply leave the EU without any agreement at all, with potentially serious consequences for the economy.
They argue that before that happens Parliament should be given a chance to vote to send ministers back to the negotiating table to try again.
The move has infuriated pro-Leave Tories who believe it is a backdoor attempt to derail the whole Brexit process.
Pro-Remainers have dismissed a claim by the leading Brexit campaigner Steve Baker that as many as 27 rebels could vote with the opposition as wide of the mark.
But with a Government majority of just 16 in the Commons, the arithmetic may be tight if it does come to a vote.
Former ministers Anna Soubry and Dominic Grieve were reported to be among the potential rebels, alongside veteran pro-European Ken Clarke, the only Tory to oppose the Bill in last week's second reading vote.
Speaker John Bercow will decide which of the scores of amendments that have been tabled MPs will have the chance to debate or vote on.
Meanwhile Labour leader Jeremy Corbyn is struggling to contain a revolt in his own ranks after 47 pro-Remain Labour MPs defied the leadership to vote against the Bill at second reading.
There was fury among MPs that shadow home secretary Diane Abbott, one of Mr Corbyn's closest allies, escaped unpunished after she failed to vote, claiming a migraine, despite a three-line whip.
Three other members of the shadow cabinet, shadow environment secretary Rachael Maskell, shadow Welsh secretary Jo Stevens and shadow equalities minister Dawn Butler, resigned to oppose the Bill.
Mr Corbyn has yet to decide what to do about 10 junior shadow ministers and three party whips who also voted against.
With the prospect of an even bigger revolt in the final third reading vote on Wednesday, possibly including shadow business secretary Clive Lewis, he has hinted he will be "lenient".
But while that may prevent the loss of further senior figures for now, it could have serious consequences for party discipline further down the line.