Theresa May will make the case for her new Brexit plan in Parliament later, amid signs that Conservative opposition to her leadership is hardening.
The prime minister will outline changes to the Withdrawal Agreement Bill - including a promise to give MPs a vote on holding another referendum.
Labour MPs said too little had changed for them to come on board.
Some senior Tories will today ask party bosses for a rule change to allow a no-confidence vote in her leadership.
MPs have rejected the withdrawal agreement negotiated with the EU three times, and attempts to find a formal compromise with Labour have failed.
Today, the British prime minister asked MPs to take "one last chance" to deliver a negotiated exit - or risk Brexit not happening at all.
But several Tory MPs have criticised her plan. Among them, Nigel Evans will today urge party bosses on the 1922 committee to change party rules to allow for an immediate vote of no-confidence in Mrs May.
Because the PM survived such a vote in December, the current rules say she cannot face another for 12 months.
The committee has said 'no' to such a change before.
Other senior Tories, including Iain Duncan Smith, have suggested Mrs may drops her Withdrawal Agreement Bill to avoid defeat and humiliation.
Conservative MP Boris Johnson - who wants to succeed Mrs May as prime minister - said on Twitter: "We are being asked to vote for a customs union and a second referendum. The Bill is directly against our manifesto - and I will not vote for it.
"We can and must do better - and deliver what the people voted for."
Meanwhile Dominic Raab, another leadership hopeful, said Mrs May's deal would "break our clear manifesto promises".