Labour must prioritise a second referendum over a general election, the party's deputy leader is to say.
Tom Watson will call on the party to back a new Brexit referendum to be held before any general election in a stand that will put him on a collision course with Labour leader Jeremy Corbyn, who has made clear his priority is for an election once Parliament has closed off a no-deal Brexit.
In a speech on Wednesday in London, Mr Watson will say a single-issue Brexit election may not break the deadlock in Parliament - something only a second referendum can achieve with certainty.
He will also argue that if a referendum were to follow an election, then Labour should commit "unambiguously and unequivocally" to campaign for Remain.
Again, his view sets him up for a clash with Mr Corbyn who is yet to say which side he would support, and has said that Labour must appeal to both Leave and Remain supporters.
Mr Corbyn's ambiguity is thought to be due to the fact that many Labour supporters backed both sides in the June 2016 referendum, and he does not want to take a firm stance and alienate one side.
The 70-year-old said an incoming Labour government would hold a new referendum - with Remain and a "credible" option for Leave on the ballot paper.
However, Mr Watson will argue it is not too late for Labour to win back Remain voters put off by confusion over the party's position on Brexit, if they were to commit clearly to campaign to stay in the EU.
"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.
"If we did it we could win, whereas if we don't I fear we won't."
Mr Watson will say that so much has changed since the original referendum in 2016 it was "no longer a valid basis" for determining Britain's future and the priority should be a new public vote before an election.
"Very difficult though it was, I and many others respected the result of the 2016 referendum for a long time," he said.
"But there eventually comes a point - and we are very far past it now, well into the fourth year since the referendum - when circumstances are so changed, when so much new information has emerged that we didn't have in 2016, when so many people feel differently to how they felt then, that you have to say, no, that years-old plebiscite is no longer a valid basis on which to take such a momentous decision about the future of the United Kingdom.
"The only proper way to proceed in such circumstances is to consult the people again."
Conservative Party chairman James Cleverly said: "Labour's deputy leader makes clear Labour want to cancel the referendum result.
"This latest trick would mean delaying Brexit again for up to a year, handing over £250 million a week to Brussels for no purpose.
"Labour are running scared of an election and only offer more dither and pointless delay.
"Only Boris Johnson and the Conservatives will deliver Brexit by October 31, no ifs or buts, so we can move on and focus on the issues that matter to people - investing in the NHS, reducing violent crime and cutting the cost of living."