Labour will not rule out including the option of staying in the EU in any future Brexit referendum, Sir Keir Starmer told the party’s conference.
The shadow Brexit secretary said all options should be kept on the table, including a so-called People’s Vote, to “stop a destructive Tory Brexit”.
Labour members are debating the controversial issue, with a vote later.
Sir Keir said Labour MPs were “likely” to be ordered to vote against any deal based on Theresa May’s Chequers plan.
Labour’s policy is to force an election if MPs are deadlocked over Brexit but members have succeeded in getting a debate on getting a fresh referendum on to the agenda at the conference in Liverpool.
Sir Keir told Labour activists the PM’s plans looked set to fail the six tests set by Labour for any deal with Brussels.
“If a general election is not possible then other options must be kept open. That includes campaigning for a public vote,” he said.
“It is right for Parliament to have the first say but if we need to break the impasse, Labour campaigning for a public vote must be an option and no-one is ruling out Remain as an option.”
The conference motion says: “If we cannot get a general election, Labour must support all options remaining on the table, including campaigning for a public vote.”
Labour leader Jeremy Corbyn – who has previously ruled out another referendum – has vowed to respect the decision of party members.
But Monday saw confusion over whether the leadership thought any fresh referendum should include staying in the EU as an option.
Shadow chancellor John McDonnell said he thought any vote should be on the terms of a Brexit deal – rather than on remaining in the EU.
But he later said “all options” were on the table – a point reinforced by Sir Keir.
The prime minister’s Chequers plan, which would see the UK staying closely aligned to the EU in some areas, such as the trade in goods, has been criticised by EU leaders and many of her own MPs.
Labour – which wants the UK to remain in a customs union but not the single market – has not ruled out voting for any deal Mrs May brings back from Brussels, ahead of the UK’s 29 March departure date.
But Sir Keir set six tests on guaranteeing workers rights and ensuring the UK continued to enjoy all the “benefits” of being a member of a customs union and single market.
Labour is publishing analysis it says will show that the PM’s proposals currently “put jobs, the economy, people’s livelihoods and no hard border in Northern Ireland at risk”.
In his party conference speech, Sir Keir said Labour’s goal was not to “frustrate the process” but to “stop a destructive Tory Brexit”.
“Some have said Labour could vote for any deal the Tories reach. Some have said we may abstain. Some have said we may vote for a vague deal.
“So, let me be very clear – right here, right now: if Theresa May brings back a deal that does not meet our tests – and that looks increasingly likely – Labour will vote against it. No ifs, no buts.
“And if the prime minister thinks we’ll can come back a vague deal asking us to leap blindfolded into the unknown, we will vote that down.”
The BBC’s political editor Laura Kuenssberg said Sir Keir’s comments meant the chances of the Brexit process “imploding” in Parliament had gone up significantly.
For the Conservatives, Brexit minister Robin Walker accused Labour of “playing political games”, adding: “Labour seem determined to take us all back to square one by rejecting a deal out of hand then trying to delay Brexit and re-run the referendum.
“This Conservative government is negotiating the right deal for the UK, one that promotes jobs and protects security and the precious Union between the four nations of the UK.”