A cross-party Brexit deal will not get through Parliament unless it is subject to a fresh public vote, shadow Brexit secretary Sir Keir Starmer says.
Talks between Labour and ministers over leaving the EU have been going on for a month with little sign of progress.
Sir Keir told the Guardian that without a new referendum, up to 150 Labour MPs would vote against any agreement made.
Foreign Secretary Jeremy Hunt said it was a “crunch week” for the talks ahead of European elections on 23 May.
He said his view had not changed that another referendum would be a “betrayal” and everyone’s focus should be on delivering on the result of the 2016 Brexit vote.
The UK was due to leave the EU on 29 March, but the deadline was pushed back to 31 October after MPs rejected Theresa May’s proposed deal three times.
Talks between the government and Labour aimed at finding a way out of the impasse will resume at 17.00 BST with pressure growing on both sides to show progress or pull out.
No 10 said there was a “clear desire” to get on with the process. Asked if there was a deadline, a spokesman said “let’s see where we get to this evening.”
If there is no agreement, Theresa May has said she will return to Parliament and ask MPs to vote again on a range of possible options.
Parliament failed to unite behind a way forward in a series of “indicative votes” in March, but the PM says the government would now be prepared to accept whatever commands a majority if Labour does so too.
Sir Keir said he would not be afraid to end the talks as soon as this week if PM did not budge on her so-called red lines – positions that she feels cannot be changed in the Brexit deal.
He suggested a referendum on the final deal had become a red line of its own for many Labour MPs, saying “a significant number, probably 120 if not 150, would not back a deal if it hasn’t got a confirmatory vote”.
Labour’s stated policy is that it supports a further referendum on Brexit under certain circumstances.
It has rejected the idea of campaigning for one in any event, but will demand a public vote if it cannot get changes to the government’s deal or an election.
Labour’s Stephen Kinnock, who backs the UK leaving but retaining the closest possible economic links with the EU, said it would be a “real shame” if the talks were “torpedoed” by his party’s insistence on another referendum.
“If you try to insert a second referendum into these talks they won’t get through because the Conservatives will not whip their MPs to support it,” he told Radio 4’s World At One.
However, Labour’s deputy leader Tom Watson said another public vote was the only “way out” of the current stalemate.
Asked whether Labour wanted to leave or remain in the EU, he told Radio 4’s Today: “We are a remain and reform party,” but “when it comes to a deal people can form their own view”.
In a speech later marking the 25th anniversary of former Labour leader John Smith’s death, Mr Watson will reflect on Mr Smith’s pro-Europeanism and say he would have backed a “People’s Vote”.
Speaking ahead of a meeting of EU foreign ministers in Brussels, Jeremy Hunt said people voted in good faith in 2016 in the belief the result would be acted upon.
“People have been pessimistic right from the outset that these negotiations were not going to get anywhere but they have continued.”
The reality is these talks have been genuine, but very difficult.
Neither side wanted to pull the plug before the local elections 10 days or so ago.
But now, as time goes on, it may well be we are reaching the moment where they have to throw up their hands and say: “We just can’t do it.”
Jeremy Corbyn and Theresa May are both on lonely tightropes trying to get across the other side of this slow-moving crisis. I think they would both like it to be over with, maybe with a cross-party deal.
But the prime minister doesn’t want to put a huge compromise on the table, she doesn’t want another referendum. Jeremy Corbyn doesn’t want to help out the government unless he can get genuine changes.
If neither of them feel they can really budge, well, the talks are not going to be able to succeed, and the government will then have to try to move on to votes in Parliament, the next part of the process.