Liberal Democrat MP Chuka Umunna signalled his party could be flexible over their date of 9 December, but said it “cannot be the 12th”.
He told BBC Radio 4’s Today programme: “We are not prepared to accept the 12th. If you have the 12th, it presents an opportunity for the Government to try and get their Withdrawal Agreement Bill through.
“We know that they have a record of going back on their promises and breaking the law, so we cannot trust them on that.”
The similar bills, one by the SNP and Liberal Democrats, and one by the prime minister, will come before the House of Commons on Tuesday afternoon.