Remaining in the EU must be an option on the ballot paper in any new referendum, Labour's shadow Brexit secretary Sir Keir Starmer said on Tuesday.
Speaking at his party's conference, Sir Keir got a standing ovation from activists as he veered away from his planned speech to say staying in the EU could not be ruled out.
Labour delegates later voted overwhelmingly in favour of the party supporting another referendum on Brexit if Parliament did not back a General Election to solve the Brexit deadlock.
Sir Keir's intervention came 24 hours after shadow chancellor John McDonnell said Remain should not be an option in any new referendum, and then claimed it could indeed be on the ballot.
In his speech in Liverpool, Sir Keir said: "Conference, it's right that Parliament has the first say but if we need to break the impasse, our options must include campaigning for a public vote and nobody is ruling out Remain as an option."
In the version of his speech sent out before he took to the stage, the line about what should be on the ballot paper was not included.
Veteran Labour MP Dennis Skinner, who has long campaigned for the UK to leave the EU, looked on stony-faced and did not take part in the raptous applause after Sir Keir's deviation from the script.
Labour MP Alison McGovern welcomed the comments, and said: "When Keir Starmer says Labour must campaign for a public vote with remain as an option and he gets a standing ovation from our party, you know just how far we've come in the past few weeks."
Labour leader Jeremy Corbyn sought to play down Sir Keir's apparent ad lib, later telling broadcasters "the speech was cleared".
The shadow Brexit secretary also used his speech to ramp up pressure on Theresa May as the negotiations with Brussels enters its final phase.
Labour has put forward six tests by which it will measure any deal she secures, with one of the criteria being any agreement must deliver the "exact same benefits" as the UK currently enjoys by being in the Single Market and customs union.
Addressing the conference, Sir Keir warned May that Labour's position was not shifting, saying: "Some say we may vote for the deal the Prime Minister brings back, maybe abstain, maybe vote for a vague deal.
"So let me be absolutely clear: If the Prime Minister returns with a deal that does not meet our tests – and that looks increasingly likely – we will vote against her deal."
While Sir Keir attempted to clear up confusion over Labour's support for another referendum, another split among the party's top team opened on whether the negotiating period with Brussels should be extended.
Speaking at a fringe event, shadow foreign secretary Emily Thornberry said a Labour government would look to secure more time for talks with the EU.
"We need to extend article 50 and essentially turn up in Europe and say: 'The grown ups have turned up now, let’s sit down and talk'," she said.
Corbyn said such a move "is not in our hands", adding: "Article 50 can only be extended by the agreement of the entirety of the European Union."