Labour will push for a new EU referendum when the government brings its Brexit plans to the Commons in the coming days.
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 "fundamentally" needed to be put "back to the people in a referendum" and said the party would back an amendment calling for one.
He told BBC One’s The Andrew Marr Show: "We need an amendment to say that whatever deal gets through, it should be subject to a referendum."
He added: "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."
He laid out Labour's plan for the week ahead to ITV News, saying step one is to "make sure that that extension is granted if we possibly can".
Secondly, he said, "there will be amendments going down if there's implementing legislation" surrounding the new deal.
He says "that will deal with the nature of the deal and to see if we can get something that's in closer alignment with the EU".
"It will also be to close the trap door to no deal which is still there for the end of 2020," he said.
"Then, fundamentally, we want to make sure that whatever deal gets through, if a deal gets through, that it is put back to the people in a referendum".
Labour is plotting ahead of another tense week in Parliament, with a Meaningful Vote on the prime minister's deal expected.
At the weekend opposition parties managed to thwart what Mr Johnson hoped would be a definitive moment in the Brexit process, when they backed an amendment to his proposed deal.
On the first Saturday sitting of Parliament in 37 years MPs voted to say they had considered the government's Brexit proposal but withheld approval of the deal until all necessary legislation had been passed.
As a result of the defeat, the PM postponed a planned vote on his deal, which will instead be put to the Commons this week.
Asked if Labour could back the Government’s deal if it came with the pledge of a new referendum, Sir Keir told Marr: “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.”