MPs are to debate and vote on the next steps in the Brexit process later, as Theresa May continues to try to get a deal through Parliament.
A series of amendments – designed to change the direction of Brexit – will be considered in the debate, which is expected to be a routine procedure.
But BBC political editor Laura Kuenssberg said the PM could be facing another defeat.
Some Tory Brexiteers are refusing to back the government, she said.
No 10 insists Mrs May still plans to hold a vote on a deal as soon as possible but Labour has accused her of "running down the clock" in an effort to "blackmail" MPs into backing her deal.
The prime minister has asked MPs to approve a motion simply acknowledging that the process was ongoing and restating their support for the approach.
But several MPs have tabled amendments – which set out alternative plans – including one from Labour that would force the government to come back to Parliament by the end of the month to hold a substantive Commons vote on its Brexit plan.
Another, from the SNP, calls on the government to pass a law leading to the Brexit process being halted.
Commons Speaker John Bercow is yet to decide which of these will actually be considered by MPs.
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However, influential Brexiteers from the European Research Group of Tory backbenchers are angry at being asked to support the PM's motion.
This is because it combines the view backed by a majority of MPs last month that the government should seek an alternative to the "backstop" with a separate move to stop Brexit happening without a formal deal.
The backstop aims to prevent the return of customs checkpoints on the Irish border in the event that no trade deal comes into force.
The group's deputy chairman, Mark Francois, told the BBC: "We cannot vote for this as it is currently configured because it rules out no deal and removes our negotiating leverage in Brussels."
He said members had "pleaded" with Downing Street to change the wording, which he said goes back on what the prime minister has previously told MPs.
"A senior ERG source says they haven't decided whether to abstain or vote against, but they won't back the government," said BBC political editor Laura Kuenssberg.