Labour deputy leader Tom Watson could back Theresa May’s Brexit deal if the government agrees to a second referendum.
Jeremy Corbyn’s deputy will set out his demand to the prime minister as the main speaker at a mass rally for a so-called people’s vote on Saturday.
The influential Labour frontbencher’s speech will mark a significant intervention in the Brexit crisis ahead of a third meaningful vote on May’s deal next week.
London Mayor Sadiq Khan, Scotland’s First Minister Nicola Sturgeon and Tory Lord Michael Heseltine are among the other speakers due to be at the rally in Parliament Square.
Watson will attack the chaos of the government’s approach to leaving the bloc and say that he has “come to the reluctant view” the only way to break the logjam is with a fresh poll.
Watson will say: “That’s why I’ve come to the reluctant view that the only way to resolve this and have legitimacy in the eyes of the public is for the people themselves to sign it off. It can only bring closure if we’re all involved in making the decision. It can only begin to bring the country back together again if we all have a final say – and then live with the result.
“So, I have an explicit message for Theresa May: I will vote for your deal or a revised deal you can agree with my party. I will help you get it over the line to prevent a disastrous no deal exit. But I can only vote for your deal - or any deal - if you let the people have a vote on it too.
“That’s why I’m proud to be marching. I trust the people I represent. And only they can sort this mess out.”
It came amid a series of developments on Friday, which included:
DUP deputy leader Nigel Dodds signalling his party would not back the PM, saying her bid to secure changes to the Northern Ireland backstop had been a “failure”
De facto deputy PM David Lidington floating the idea of an X Factor-style run-off votes on Brexit options should May’s deal fail
A petition for the government to revoke Article 50 and remain in the EU collecting almost 3.5m
Watson will make his case after the Labour leadership came under significant pressure from its pro-EU membership to press for a fresh vote.
Backbench MPs Phil Wilson and Peter Kyle are set to table an amendment to May’s Brexit plan, saying MPs will support May’s deal on the basis of a confirmatory referendum.
It is unclear whether Labour will whip its MPs to back it, however, amid fears Corbyn could face a walkout of frontbenchers, many of whom represent strongly pro-Leave constituencies and are against a re-run of the Brexit poll.
But in what appears to be an appeal to them, Watson will say: “Everyone in this country has been let down by this Brexit process; people who voted to leave, people who voted to stay, people who didn’t vote or were too young to vote.”
He will add: “So now, at the 11th hour and the 59th minute, parliament has now got to get this right for everyone.”