Labour is promising to campaign for Remain in a new EU referendum against “no deal or a damaging Tory Brexit”.
The shift in stance was signalled by Labour leader Jeremy Corbyn, as he challenged the next prime minister “to put their deal to the people”.
But the statement does not explain what Labour would do if the party won a general election.
Jeremy Corbyn has faced calls to move policy in a more pro-EU direction from senior members of Labour.
In a letter to members, the party leader said: “Whoever becomes the new prime minister should have the confidence to put their deal, or no deal, back to the people in a public vote.
“In those circumstances, I want to make it clear that Labour would campaign for Remain against either no deal or a Tory deal that does not protect the economy and jobs.”
The announcement follows a shadow cabinet meeting on Tuesday morning, and a meeting with trade union leaders on Monday.
The bosses of Labour’s five-biggest affiliated unions called for the move the party has made – but also for it to hold a “confirmatory vote” on any new deal it negotiated if Labour won a general election.
According to the BBC, there was disagreement about the second part of the unions’ stance in the shadow cabinet meeting, with deputy leader Tom Watson wanting a “straight Remain stance”, meaning a decision on it was “kicked down the road”.
Mr Watson is among other leading figures who have called for an unambiguously pro-Remain stance amid criticism that confusion over Labour’s message contributed to its poor performance in the recent European Parliament elections.
But MPs from Leave areas of the UK have warned it could damage the party’s next election performance.
In his letter, Mr Corbyn said Labour continued to believe the “compromise plan” set out for Brexit during cross-party talks with the government earlier this year was still a “sensible alternative that could bring the country together”.
This included a customs union, a strong single market relationship and the protection of environmental regulations and rights at work.
He renewed calls for a general election in the letter, saying Labour had a “crucial, historic duty to safeguard jobs, rights and living standards, but no Brexit outcome alone can do that.”
But Mr Corbyn did not say if Labour would pursue a new deal or a further referendum if the party won a general election.