Talks between Conservative and Labour teams are taking place for a second day as they try to end the Brexit deadlock.
It follows discussions between Theresa May and Jeremy Corbyn on Wednesday, which were described as “constructive”.
Shadow Brexit secretary Sir Keir Starmer said the idea of a “confirmatory” referendum on any Brexit deal would be discussed.
MPs backed a bill, which would force the PM to seek a new delay to Brexit. Peers are due to debate it later.
The UK is due to leave the EU on 12 April, and as yet, no withdrawal deal is in place.
Ministers have warned the backbench bill – put forward by Labour’s Yvette Cooper – could increase “the risk of an accidental no-deal”.
No 10 says the bill, which scraped through the Commons with a majority of one vote on Wednesday, would deny the PM the power to agree a deal with EU leaders on April 10 – two days before exit – as MPs would have to agree to any new Brexit date.
Any Brexit delay will require the unanimous backing of all 28 EU leaders at a summit next Wednesday.
If they agree but suggest a different date to the one backed by MPs – the prime minister would have to bring it back to the Commons for further approval on Thursday 11 April.
“By April 11, the European Council will have concluded and the leaders will have returned to their member states. In the words of the secretary of state, the bill could increase the risk of an accidental no-deal exit,” the prime minister’s spokesman said.
The backbench bill will need the approval of the House of Lords if it is to become law.
The Lords is debating the procedure to bring forward the bill, before discussing the bill itself.