A cross-party Brexit deal will not get through Parliament unless it is subject to a confirmatory vote, shadow Brexit secretary Sir Keir Starmer has said.
Talks between Labour and ministers over quitting the EU continue on Monday.
But Sir Keir told the Guardian that, without a new referendum, up to 150 Labour MPs would vote against any deal.
And he added he would not be afraid to pull the plug on cross-party talks as soon as this week if the prime minister did not budge on her red lines.
Theresa May is hoping to reach a cross-party consensus on her withdrawal agreement after failing to get it through Parliament three times.
Sir Keir said: “A significant number of Labour MPs, probably 120 if not 150, would not back a deal if it hasn’t got a confirmatory vote.”
“If the point of the exercise is to get a sustainable majority, over several weeks or months of delivering on the implementation, you can’t leave a confirmatory vote out of the package.”
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Meanwhile, in a speech later, Labour’s deputy leader Tom Watson is set to “plead” with voters to back Labour in the European Parliament elections – despite the party’s Brexit stance.
Campaigning for the European elections continues – with votes to elect 73 UK MEPs due to be cast on 23 May – and Mr Watson will warn voters not to turn to the “nasty nationalism of the Farage Brexit Party”.
Mr Watson’s speech will mark the 25th anniversary of the death of the former Labour leader John Smith.
In it, he will say he believes Mr Smith would have supported the view of his former deputy, Dame Margaret Beckett, and backed a “People’s Vote”.
Mr Watson will also say that “corrosive far-right sentiment, posing as patriotism, will dominate this EU election”.
“There are only two forces that can win this election – that nasty nationalism of the Farage Brexit Party, or the tolerant, compassionate outward-looking patriotism of the Labour Party.
“I can only plead with Labour supporters – don’t stay at home, don’t put that cross elsewhere, don’t let them win.”
Mr Watson had called for his party to promise a referendum on any Brexit deal in its European elections manifesto.
The UK had been due to leave the EU on 29 March, but the deadline was pushed back to 31 October after Parliament was unable to agree a way forward.
Ahead of European elections later this month, two separate polls, by ComRes and Opinium, give the Brexit Party the biggest share of the vote with the Conservatives in fourth place behind Labour and the Lib Dems.
Labour has agreed to support a further referendum on Brexit under certain circumstances.
Its governing National Executive Committee, when it met to decide the wording of the EU election manifesto, rejected the idea of campaigning for a referendum under all circumstances.
But the party will demand a public vote if it cannot get changes to the government’s deal or an election.
Labour’s position on Brexit
June 2017 – Labour’s general election manifesto accepts referendum result
March 2018 – Shadow Northern Ireland secretary Owen Smith sacked for supporting second referendum on final deal
September – Labour agrees if a general election cannot be achieved it “must support all options… including a public vote”
18 November – Labour leader Jeremy Corbyn says a new referendum is “an option for the future” but “not an option for today”
28 November – Shadow chancellor John McDonnell says Labour will “inevitably” back a second referendum if unable to secure general election
16 January 2019 – 71 Labour MPs say they support a public vote
6 February – Mr Corbyn writes a letter to Mrs May outlining five changes with no mention of a “People’s Vote”
28 February – Labour says it will back a public vote after its proposed Brexit deal is rejected
14 March – Five Labour MPs quit party roles to oppose a further referendum
27 March – The party backs a confirmatory public vote in Parliament’s indicative votes on a way forward for Brexit
30 April – Party agrees to demand a public vote if it cannot get changes to the government’s deal or an election, as it decides wording to EU election manifesto