Mr Johnson said he wanted to head to the polls next month to break the political deadlock, and accused opposition parties of making "outrageous excuses" to delay a vote.
While stopping short of calling for that himself, Mr Watson will say it is not too late for Labour to "win back" Remain voters from the Lib Dems if it campaigns "unequivocally" to remain.
"And that is why Jeremy was right to say at Congress yesterday that an incoming Labour government will commit to a referendum".
The UK prime minister chose to send MPs home through October 14, but first tried to call a snap election to ensure his parliamentary majority, but the House of Commons twice opposed that proposal.
'And I can tell you this: We're ready for that election.
Mr Corbyn threatened to "unleash the biggest people-powered campaign we've ever seen", when addressing union members.
"We can't ask people to vote to renegotiate then might ask the public to vote that down", he tweeted.
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Corbyn later met leaders of the key Labour-affiliated unions, and agreed the party should stick to its policy of backing a referendum, but ensuring they have something to offer leave supporters. "If Labour is in government it will negotiate the best possible deal it can and let people decide between that deal and the current deal as European Union members", said one person with knowledge of the meeting.
But Mr Johnson later dismissed as a "load of nonsense" accusations that suspending Parliament is "anti-democratic", insisting it was necessary to prorogue before the Queen's Speech. "Just as we worked throughout the summer to pass a law preventing no deal, so we will work each and every day we are shut down to enforce that law. And I think that's unbelievable, that decent people who have devoted their entire lives to mainstream politics are being driven out by this culture of extremism and in some cases, anti-semitism.I have to be honest and the truth is that I have become ashamed of the Labour Party under Jeremy Corbyn".
If Labour is to regain the insurgent momentum of 2017 the focus of Corbyn and shadow minister of labour Laura Pidcock's speeches to TUC show us where to start. He claimed Johnson's government is not serious about trying to get a Brexit deal with the EU.
"We will put power in the hands of workers".
The PM had already kicked off his unofficial election campaign with a visit to Yorkshire on Thursday, during which he re-announced his police recruitment drive.
McDonnell suggested on Sunday the party would not seek to open lengthy negotiations with Brussels, but would be hoping to rubber-stamp something like Theresa May's deal, with the addition of the compromises Labour had secured during the cross-party talks earlier this year.
"So a general election is coming".