The line was not signed off by Jeremy Corbyn’s office and comes after Sir Keir found himself at loggerheads with one of the leader’s closest allies over the issue.
The shadow Brexit secretary was given a standing ovation by Labour delegates in the conference hall as he made clear that any future referendum backed by the party could have an option to stay in the EU.
But he was immediately contradicted from the stage by a senior figure in the Corbyn-backing union Unite, while Labour Leave, which campaigned to quit the EU, branded it a “betrayal”.
Labour conference will on Tuesday vote to back a motion painstakingly negotiated by the shadow Brexit secretary, which says that if Theresa May’s plans are voted down in the Commons and no general election is called, Labour “must” campaign for a new referendum.
Speaking to conference, Sir Keir said that if Labour could not secure a general election, “we must have other options”.
He went on: “That must include campaigning for a public vote.
“It is 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.”
The final words of the sentence were not included in the copy of the speech pre-briefed to journalists, while insiders said that it had been ad-libbed by the frontbencher from the stage.
With all speeches having to be cleared with Mr Corbyn’s office first, it will mean that the words were not approved by the leader’s aides.
A source in his office did not steer The Independent away from the suggestion that the leader had no prior knowledge of the comments, but did confirm the substance of them, saying that it was a “statement of fact”.
Labour’s Alison McGovern MP, leading supporter of the People’s Vote, said: “Keir Starmer’s speech shows that the Labour leadership listens to Labour voters and Labour members. Labour voters, supporters and members all reject Theresa May’s Brexit, Jacob Rees-Mogg’s Brexit and any idea of a blindfold Brexit.”
Mr McDonnell had said that Labour would “go for a people’s vote” on leaving the EU, but any referendum would not include an option to stay in the bloc – but he was then forced to correct himself after he was contradicted by Sir Keir in an interview.
Mr McCluskey had said that it would be “wrong” to have a Remain option in a future referendum, and was on Tuesday backed by his union’s assistant general secretary.
Steve Turner took to the conference stage soon after Sir Keir’s speech to say: “Despite what Keir said earlier, it’s a public vote on the terms of our departure.”
Sir Keir also confirmed in his speech that the party is likely to vote down the deal that Theresa May brings back from Brussels, if she does secure one, because it does not meet Labour’s “six tests”.
Brendan Chilton, general secretary of Labour Leave, said of the speech: “This is a betrayal of the very highest order. It is a betrayal not only of the millions of Labour voters, but of our 2017 manifesto.
“It is a betrayal that voters will remember for a very long time, and we will lose MPs as a result. There is no doubt. It was a P45 to our MPs in the Midlands and Wales.”
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies