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 give a statement to MPs on her changes to the Withdrawal Agreement Bill – including her promise to give MPs a vote on holding another referendum.
But Labour MPs said too little had changed for them to come on board.
And one ex-Tory minister questioned whether June’s vote would even happen.
MPs have rejected the withdrawal agreement negotiated with the EU three times, and attempts to find a formal compromise with Labour have failed.
On Tuesday, the prime minister asked MPs to take “one last chance” to deliver a negotiated exit – or risk Brexit not happening at all.
But Mrs May’s new plan had “failed to turn sceptics into endorsers”, said BBC political correspondent Chris Mason.
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”.