Labour’s divisions on Brexit will be laid bare on Wednesday, as Tom Watson urges his party to “unambiguously and unequivocally back remain” – and to push for a referendum, before a general election.

Speaking at a Creative Industries Federation conference in London on Wednesday, Watson will argue that Labour can win back the remain voters who deserted it in May’s European elections; but only if it now becomes an avowedly anti-Brexit party.

“Boris Johnson has already conceded that the Brexit crisis can only be solved by the British people. But the only way to break the Brexit deadlock once and for all is a public vote in a referendum. A general election might well fail to solve this Brexit chaos,” Watson will say.

If there is a general election before a referendum, he will say, “I will be arguing that our position going into that election should be totally clear – we must unambiguously and unequivocally back remain”.

Watson will insist that it is not too late to win back remainers attracted by the Lib Dems’ simpler “stop Brexit” message.

“My experience on the doorstep tells me most of those who’ve deserted us over our Brexit policy did so with deep regret and would greatly prefer to come back; they just want us to take an unequivocal position that whatever happens we’ll fight to remain, and to sound like we mean it,” he will say.

Watson will say: “There is no such thing as a good Brexit deal, which is why I believe we should advocate for remain”

His stance contrasts with Jeremy Corbyn’s insistence on Tuesday that any referendum must include a “credible leave option,” leaving the door open for Brexit voters to support Labour.

Addressing the Trade Union Congress in Brighton, after twice whipping his MPs not to support Boris Johnson’s bid to trigger a snap general election, Corbyn said, “a general election is coming. But we won’t allow Johnson to dictate the terms.

“And I can tell you this: we’re ready for that election. We’re ready to unleash the biggest people-powered campaign we’ve ever seen. And in that election we will commit to a public vote with a credible option to leave and the option to remain.”

He warned that Johnson was pursuing a “Trump deal Brexit”, and accused him of, “hijacking the referendum result to shift even more power and wealth to those at the top”.

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.

“The policy is clear – if it is a bad Tory deal then Labour will formally back remain. 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 EU members,” said one person with knowledge of the meeting.

Shadow Brexit secretary Keir Starmer will echo Corbyn’s language about a “credible leave option,” when he addresses the TUC on Wednesday.

He will promise that MPs won’t be “silenced” by Johnson’s decision to prorogue parliament. “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.

“Prime minister: you can hide from parliament for a few weeks, but when we return we will be ready,” Starmer will say.

Watson’s intervention is likely to be viewed as less than helpful by the Labour leadership.

One Corbyn loyalist in the shadow cabinet said, “Tom has been increasingly detached from mainstream Labour thinking. He often misses shadow cabinet these days. This speech does not reflect Labour policy in any way. Many on the left of the party are suggesting that he wants to avoid an election at all costs out of fear that Jeremy may become prime minister.”

By contrast to Watson’s claim, party strategists believe the most ardent remain voters will be all but impossible to win back – but hope they can offer a pragmatic way out of the current crisis for both leavers and remainers.

There was disquiet among some around Corbyn that the chaotic events of the past week in parliament risked allowing Johnson to present the Labour leader as part of a Westminster cabal, trying to block Brexit.

But they hope that once a general election campaign kicks off, focus will switch to Corbyn’s radical policies, including the increase in workers’ rights he outlined in Brighton on Tuesday. “Once we get out of here, there isn’t any way to establishment-up Jeremy,” one Labour source said.

Labour’s Brexit policy has shifted in a series of incremental steps over the past 12 months, from a referendum being one option “on the table”, to its preferred route out of the deadlock in parliament.

Some members of the shadow cabinet, including Emily Thornberry and John McDonnell, have already made clear they would campaign to remain in a referendum, whatever deal Labour was able to secure from the EU27.

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.

These included continued customs union membership until the question of whether to leave could be settled at a general election; and closer alignment with the EU on workers’ rights and environmental standards.