Labour must prioritise reversing Brexit through another referendum, over winning power in a general election, its deputy leader Tom Watson is to say.
He will warn that a snap election before the end of the year may fail to resolve the current deadlock.
Putting himself at odds with Jeremy Corbyn, he will say there is “no such thing as a good Brexit deal” and Labour must campaign unequivocally to remain.
Mr Corbyn has said Labour must offer voters both Leave and Remain options.
Following meetings with trade union leaders on Tuesday, Labour’s leader promised a further referendum on Brexit with a “credible Leave option” versus Remain if his party wins the next general election.
Its election manifesto will promise to reach a better Brexit deal, but is not expected to commit to either Leave or Remain.
Mark Serwotka, the general secretary of the PCS union, said this was the “only common sense” position and the binary choice on offer in a referendum was a “huge gamble” which risked “perpetuating” existing political divisions.
“I think Tom Watson’s intervention is irresponsible and not what Labour communities need,” he told the BBC’s Newsnight.
Mr Watson and other senior Labour figures, including shadow Brexit secretary Sir Keir Starmer, say they will campaign to stay in the EU in any circumstances.
BBC Newsnight political editor Nick Watt said he believed Mr Corbyn’s aim was to “provide a home” for both Leave and Remain supporters at the next election but he could be forced to allow colleagues to campaign on both sides in the event of another referendum.
Labour has voted twice against Boris Johnson’s plans for a poll on 15 October.
The party’s leadership has insisted it is eager for an election after the risk of a no-deal Brexit on 31 October has been ruled out.
In a speech in London, Mr Watson will say that while an autumn general election seems inevitable given Boris Johnson’s loss of control in Parliament “that does not make it desirable”.
“Elections should never be single issue campaigns,” he will say, suggesting vital issues such as the future of the NHS, economic inequality and crime will be “drowned out” by Prime Minister Boris Johnson’s “do or die” Brexit message.
“The only way to break the Brexit deadlock once and for all is a public vote in a referendum,” he will say. “A general election might well fail to solve this Brexit chaos.”
In the event of another general election in the coming months, Mr Watson says Labour must be “crystal clear” about where it stands on Brexit if it wants to get a hearing for the rest of its domestic policy agenda.
“There is no such thing as a good Brexit deal, which is why I believe we should advocate for remain.
“That is what the overwhelming majority of Labour Party members, MPs and trade unions believe.”
The Liberal Democrats, who pushed Labour in to third place in May’s European elections with a strident anti-Brexit message, are pushing for Brexit to be stopped in its tracks by revoking Article 50 – the legal process for the UK’s departure.
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.
“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,” he will add.
“They just want us to take an unequivocal position that whatever happens we’ll fight to remain, and to sound like we mean it.”
Mr Watson will say that, though “very difficult”, he and many others “respected the result of the 2016 referendum for a long time”.
“But there eventually comes a point,” he will add, “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… the only proper way to proceed in such circumstances is to consult the people again.”
Speaking to the Trades Union Congress on Tuesday, Sir Keir Starmer will say that another referendum is the only way of “cleaning up the mess left by the Tories”.
“And – of course – ‘remain’ should and will be on the ballot paper along with a credible option to leave.”
The Conservatives said Mr Watson had made it clear he wanted to “cancel” the 2016 Brexit referendum result.
“Only Boris Johnson and the Conservatives will deliver Brexit by 31 October, no ifs or buts, so we can move on and focus on the issues that matter to people,” said its party chair James Cleverly.