Tom Watson has claimed that on any Brexit deal "there will be a referendum" and says the Labour Party must campaign to remain or else it will be "electoral history".
The deputy Labour leader confirmed to ITV News Political Editor Robert Peston that Jeremy Corbyn was moving toward backing a "referendum in all circumstances" but said the party had not decided which way it would campaign.
His comments came after a shadow cabinet meeting which failed to go as far in shifting the party’s position as pro-EU campaigners in the Labour ranks had hoped.
Deliberations will continue until Tuesday as Mr Corbyn consults union chiefs and Labour’s National Executive Committee, Mr Watson said.
Mr Watson told Peston's Wednesday evening ITV programme: "If we do end up deciding we are going to campaign to remain in the EU it’s history making.
“I think that’s quite important because we could be electoral history if we don’t make a clear statement of intent.”
He added: “I think it is in the national interest that we become a pro-remain, pro-reform party and we let the people have a final look at what any deal looks like.”
When pressed on whether Labour would push to remain in the EU Mr Watson said "it would be impossible for us not to".
He did suggest party leader Corbyn, who was a long time Eurosceptic before the EU referendum in 2016, may play a background role in campaigning.
He said it "wouldn't be unreasonable" to assume Mr Corbyn would allow his MPs to campaign whichever way they wanted.
Despite that he did say he believed "the party should have a strong position so we can be clear with the country."
He added: "We were right in the early days to reserve judgement to see how the negotiations went, to see what the priorities would be but now (voters) expect us to give an honest account of ourselves."
Also appearing on the programme was the latest casualty of the Tory leadership race, Rory Stewart, who was eliminated in the contest's third ballot.
Peston suggested it was his decision to position himself as an alternative to the other four during Tuesday evening's TV debate that made him appear too divisive for Tory MPs to back him.
Mr Stewart admitted he was worried about being too divisive and said that as a result he was "slightly pulling my punches".
He added: "I kept being tempted to keep returning and saying, to Boris (Johnson), for example, 'how are you going to do this, how are you going to do that?'.
"In the end I became a slightly odd combination of slightly polite and slightly distanced from the debate."
He claimed most of his lost votes "will have gone to Michael Gove, Jeremy Hunt and some of them to Sajid" and denied he had been the victim of any Boris Johnson voting strategies.
Peston asked whether the moment he put his head in his hands around two thirds of the way through the debate was the moment he felt the contest slipping away.
Mr Stewart responded: "Yes, I probably felt that I wasn't managing to communicate truth properly."
He added: "Although all the polling from YouGov suggested the audience broadly agreed with me, four out of the five people on stage disagreed with almost every word that I was saying."
He refused to confirm who, if anyone, he would be backing in Thursday's ballots, but said the way to beat frontrunner Johnson is "by asking questions".
He said the question that Mr Johnson needs to be asked is which side of the Conservative party he would be prepared to let down - suggesting the former foreign secretary had made different Brexit pledges to both.
He said: "If someone's able to ask him that question something quite interesting might happen and probably it was a mistake of me to be too polite to ask that question."