On the eve of the huge Put It To The People march, Mr Watson has broken with his leader to declare that any exit plan – whether Theresa May’s or Labour’s – will have “legitimacy” only with the public’s endorsement.
The move significantly increases the pressure on Mr Corbyn to finally end months of uncertainty over Labour’s position on a new referendum, ahead of another meaningful vote on the prime minister’s deal next week.
Mr Watson also revealed he will speak in front of hundreds of thousands of expected marchers in London on Saturday – while both Mr Corbyn and shadow chancellor John McDonnell stay away.
“The current impasse is not working for people who voted to leave or people who voted to stay. I really don’t think parliament will be able to resolve this,” Mr Watson said.
“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 begin to bring the country back together again if we all have a Final Say – and then live with the result.”
Crucially, Mr Watson offers support to pass the prime minister’s deal, subject to confirmation in a referendum – increasingly its only chance of approval by the Commons.
The Labour leadership has hinted it will support the Kyle-Wilson amendment to achieve that, but is still wrangling over the wording and whether to whip its MPs behind it.
Mr Watson added: “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.”
Until now, Mr Watson has stuck to the party line of preferring a softer Brexit, saying Labour was “moving in the direction” of backing a referendum only if its own plan had to be abandoned.
To underline the widening chasm between Mr Corbyn and his number two, the announcement came as Labour tabled an amendment to secure Commons time next week to debate all options.
Furthermore, Mr McDonnell said he would not be marching on Saturday because to do so would “alienate” people who backed Leave three years ago.
“By going there, I might alienate some of the people who are strong Leavers that I want to bring on board,” he told Channel 4 News.
Asked if he personally supported another referendum, the shadow chancellor replied: “Um,” before adding he wanted to secure “a compromise that will hold”.
Mr Watson will be a star attraction at the march, beginning at 12pm on Park Lane and finishing with a mass rally and speeches in Parliament Square.
The People’s Vote campaign, which has organised the event, is predicting a high turnout with more than 200 coaches and even a specially chartered train bringing in demonstrators.
The Independent’s petition calling on the government to give the people a Final Say on Brexit has attracted more than 1.1 million signatures.
Among the speakers will be Rania Ramli, of the pro-referendum students’ group For our Future’s Sake (FFS) and national chair of Labour Students, who said: “I’m marching because the government is in chaos, Brexit is a disaster and it’s my future that’s at stake.
“Like so many students and young people, I was too young to vote in 2016 and so much has changed since.”
Former prime minister Tony Blair urged Theresa May to recognise that the march showed she need to listen to voices other than the Brexiteers in her own party in order to “escape from the chaos”.
“This weekend a huge rally will take place in London in support of a people’s vote,” he wrote in the Evening Standard.
“It will be a reminder to all MPs that the Brexiteers are not the only section of opinion that matters.”
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