Ex-trade union leader Len McCluskey believes Labour faces an “almost impossible” challenge to win the next general election as he spoke of his pessimism that Sir Keir Starmer could even force a hung parliament in 2024.

Speaking at a fringe event at Labour’s conference, the former Unite general secretary accused Sir Keir of a “poor” performance during his almost 18 months as Labour leader.

And he claimed Sir Keir had “told lies” and “created division”, as he urged Labour members to block their leader’s plans to overhaul the party’s rulebook during the Brighton conference.

Mr McCluskey stood down as Unite general secretary this summer after more than 10 years as the head of the union, which is one of Labour’s biggest financial backers.

He said he had been prepared to treat Sir Keir “at face value” following his election as Labour leader last April.

But he claimed that Sir Keir had “lost my trust and my friendship” after the pair fell out following the suspension of former party leader Jeremy Corbyn as a Labour MP as part of an antisemitism row.

“Keir has told lies ever since,” said Mr McCluskey, who is known as “Red Len” and is a political ally of Mr Corbyn.

And, on Sir Keir’s chances of becoming prime minister at the next general election, he added: “It’s almost impossible for Labour to win the election outright, the maths just don’t add up especially when you look at Scotland.

“Unless Scottish Labour do something dramatic, we have lost Scotland for a generation.

“There is still a possibility of a hung parliament if he [Sir Keir] wins back the ‘red wall’ seats. But he can only do that with radical left economic policies and other issues and with organised labour – the trade unions.

“The way he’s gone about it… I’m afraid I am very, very pessimistic indeed. So do I want him to win the next election? Of course I do. Anything is better than a Tory win. But I am deeply, deeply concerned.”

Mr McCluskey claimed he was unsure whether Sir Keir was a naive “babe in the woods” or whether he was “some cynical manipulator” who had been “captured by more sinister forces”.

The start of Labour’s conference in Brighton has been marked by a row over Sir Keir’s bid to alter the rules on how the party elects future leaders, including a proposed abandoning of the system that saw Mr Corbyn twice elected to the party’s leadership.

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Sir Keir has since been forced to water down his plans but will still propose some rule changes to Labour members in Brighton.

However, Mr McCluskey urged those at the conference to oppose Sir Keir’s efforts to change the Labour rulebook.

“Nobody is fooled by rule changes,” he said. “The rule changes are designed to try and reduce ordinary members’ input into what’s happening.”

Claiming that the “establishment both inside and outside Labour Party.. will have had, over their cigars and brandies and private clubs, apoplexy at the thought that Corbyn almost got to the levers of power”, he added: “They will have made it abundantly clear that it can never, ever happen again.

“So what is being attempted in terms of the rule changes is – if Starmer loses an election and has to go – that a Left leader will not emerge.

“The rule changes should all be opposed.”