Unite leader Len McCluskey insists he is “now full of optimism” about Labour’s general election hopes despite saying in an interview he could not see the party winning.
The union boss had told Politico a Labour victory would be “extraordinary” and suggested winning just 200 seats would be a “successful” result.
But speaking on Wednesday morning, he distanced himself from the comment.
Labour launched its general election manifesto on Tuesday.
Mr McCluskey, one of Labour leader Jeremy Corbyn’s closest allies, had told Politico that, while he supported the manifesto proposals, he was not “optimistic” about Labour’s chances on 8 June, given the hostility which he said it faced in large sections of the media.
But on Wednesday morning he said the interview had been a “conversational piece” against the backdrop of “if the opinions polls are to be believed”.
“I am now full of optimism. If I was having that interview now I would not be making those comments,” he said.
Mr McCluskey said he was now convinced the polls would change and that Labour was “in with a real chance”, describing the party’s campaign as “brilliant”.
The BBC understands the Unite leader spoke to Politico website at 12:02 BST on Tuesday – an hour after the Labour manifesto was published – and that he had attended the party meeting where the proposals were agreed.
He was quoted saying winning the general election would be a “huge task” given the “imagery of Jeremy”.
He told the website: “He’s got now just under four weeks to try to see if you can break through that image and it’s going to be a very, very difficult task… whether that breakthrough can happen, we’ll wait and see.”
He added: “People like me are always optimistic… things can happen. But I don’t see Labour winning.”
He went on to suggest that if Labour emerged with 200 seats – which would be about 30 fewer than Ed Miliband secured in 2015 – it would represent a “successful campaign” given the circumstances it found itself in, despite the fact that in terms of seats it would be Labour’s worst result since 1935.
“It will mean that Theresa May will have had an election, will have increased her majority but not dramatically,” he said.
After the original interview was published, another union leader, Unison’s Dave Prentis, tweeted that success equalled a Labour government, adding: “That’s what care workers, nurses and teaching assistants need.”
Mr McCluskey’s comments came as Labour and the Conservatives clashed over the cost of Labour’s manifesto pledges.
The Conservatives said bringing the National Grid and the water industry back into public ownership – which would require a future Labour government to compensate existing shareholders – would add £14bn to the national debt in one year alone. They said there was a £58bn “black hole” across all of Labour’s manifesto plans.
Labour said this was “absolute rubbish” and that the Tories were trying to “avoid scrutiny of their own spending plans”.
The party, which has also pledged to nationalise the railways and Royal Mail, said it could be done at no net expense to the public purse and that consumers would see their utility bills come down by as much as £220 a year.
Chancellor Philip Hammond led the Conservative attack on the proposals, describing the manifesto as a “blueprint for crashing the economy”.
Labour said it was “extremely worrying” that Mr Hammond “can’t tell the difference between capital spending and revenue spending”, adding that the Tories had “failed on every fiscal target they have set themselves”.
On its utilities nationalisation plans, it said it would exchange government bonds for shares in the companies – which would mean extra borrowing – but, since profits currently used to pay dividends would be used instead to pay interest on those bonds, there would be “no net cost” to the exchequer.
Mr Hammond defended the Tories’ calculations, saying capital investment would need to be paid for either by increasing taxes or borrowing.
He also said reports of tensions between him and Downing Street were “media tittle tattle”.
Asked about reports of swearing and angry phonecalls, he laughed and said: “I’m not going to say I’ve never occasionally sworn but I work extremely well and closely with Theresa May’s team.”