And shadow Brexit secretary Sir Keir Starmer left open the possibility the party could back Boris Johnson's deal if a new national poll was attached to it.
Sir Keir insisted that any agreement approved by Parliament needed to be put to voters in a referendum and the party would back an amendment calling for one.
The Shadow Brexit Secretary told the BBC: "Of course we need an amendment to say that whatever deal gets through, it should be subject to a referendum.
"Where that deal is put to the public and they’re asked 'do you want to leave on these terms, or would you rather remain in the EU'."
Sir Keir said any amendment would most likely come from campaigning backbench MPs - such as Labour's Peter Kyle and Phil Wilson.
Asked If Labour would then back it, he replied: "Yes. Almost every victory we've had on anything in the last three years has come from the back benches."
"We have already voted, I think, three times as a party for a second referendum with a three-line whip behind it.
"The position we have adopted is whatever the outcome, whether it's Boris Johnson's bad deal or a better one which could be secured, it has got to go to a referendum up against remain."
Asked if Labour could back the Government's deal if it came with the pledge of a new referendum, Sir Keir said: "Well, we'll see what that looks like.
"What we are trying to achieve is that this deal in particular, but any deal, is put up against Remain in a referendum.
"And we will have to see tactically how we get there."
Sir Keir said Labour would also push for a customs union.
He said: "We have been arguing for a very long time now for a customs union with the EU and for single market alignment."
John McDonnell, the shadow chancellor, confirmed to Sky’s Sophy Ridge that Labour would back a second referendum amendment, probably proposed from the backbenches.
Read more from Yahoo UK:
He said Johnson “may well be in contempt of parliament or the courts” for having sent an additional letter to the EU arguing against a Brexit extension.
“Not signing the [extension request] letter? He’s behaving a bit like a spoiled brat,” he said.
Dominic Raab, the foreign secretary, was optimistic that Johnson had enough support to get his deal passed.
“We seem to have the numbers in the House of Commons. Why hasn’t parliament pushed this through? That’s what we’re going to do this week,” he told the Marr show. “We’ve got a deal. Why would we have a second referendum?”
Michael Gove, the Cabinet Office minister in charge of no deal planning, insisted the UK would be leaving on 31 October and an extension would not be necessary.
Watch the latest videos from Yahoo UK