Firebrand union leader Len McCluskey has fired a vicious parting shot against Sir Keir Starmer, claiming Labour’s ship could  “go under” with him at the helm.

In hard-hitting memoirs to mark his retirement, the Unite general secretary known as “Red Len” suggests Sir Keir will fail to win back the “Red Wall” seats Labour lost to the Tories in 2019.

Mr McCluskey, still Jeremy Corbyn‘s chief union cheerleader, accuses the Labour leader of struggling to inspire the public, “vapid New Labour cliches”, and an “anti-democratic crackdown on the left”.

In the book, titled Always Red, he settles old scores with enemies in the Labour movement including Tom Watson and Gerard Coyne – the rival he narrowly defeated in an election in 2017 who is now a candidate for general secretary once again.

And Mr McCluskey also confirms one of Westminster’s worst kept secrets: that he is in a relationship with Mr Corbyn’s chief of staff Karie Murphy, who is singled out for praise several times in the book.

Writing about rumours about their “personal connection” when she went to work for Mr Corbyn, he admits to “a game of cat of mouse” with the press. “We wanted our relationship to be kept private, away from the public gaze,” he reveals.

But in the very last sentence in the book, he writes: “Finally, thank you to Karie for persuading me to write this book and giving me the love and support to see it through.”

In a highly unusual move, several pages of what the publishers call “sensitive material”, including a showdown with Sir Keir about Mr Corbyn’s suspension from Labour, have been redacted until publication next month.

An uncompromising and highly critical chapter about the Labour leader, in which Mr McCluskey questions whether he is a “babe in the woods” or “altogether more calculating”, includes no fewer than 25 blacked out paragraphs.

The first redaction is about a phone conversation when Sir Keir told the Unite leader he had suspended Mr Corbyn over antisemitism, followed by more than two pages about a meeting with the leader and his deputy Angela Rayner.

Mr McCluskey writes: “Angela began by requesting our discussion be confidential. Given what happened subsequently, I no longer feel bound by that.”

Then comes the two-page redaction. And he adds defiantly: “I am so confident of the account I have given here that I have submitted it for use in legal proceedings and will stand by it in court.”

In a withering verdict on Sir Keir after Mr Corbyn’s suspension, Mr McCluskey writes: “I still hoped and believed that Keir Starmer could be Prime Minister.

“But I was fearful that if he continued on the course set in his first year of leadership he would not win back the red wall seats.

“If a general election was called early, which seemed possible, Starmer would have little time to rectify his mistakes.

“He still had the opportunity to change course, unite his party around a radical platform and make the promised ‘moral case for socialism’.

“But he needed to realise that if the ship he was captaining listed too far to the right, it would go under.”

On his feud with Mr Watson, Mr McCluskey talks about “the death of a friendship” when the then Labour deputy leader pulled out of peace talks with the unions during a coup against Mr Corbyn in 2018.

He adds: “I haven’t spoken to Tom since, save for a few barbed texts two years later when his attempt to unseat me as Unite’s general secretary failed.”

Accusing Mr Watson of “a view to doing Corbyn in” and being “duplicitous”, Mr McCluskey speculates that his motives may have been to become interim leader and maybe stay on without a contest.

“Whatever the truth, it was a squalid, ignoble way to end a valued friendship.”

On Mr Coyne, Mr McCluskey says; “There is no doubt that the 2017 Unite general secretary election was a proxy war. If they could take me down, they could take Jeremy down.

“Were it not for the group of right-wing MPs and officials sometimes dubbed the ‘West Midlands mafia’ – the likes of John Spellar, Jess Phillips and Tom Watson – I’m sure Coyne would have remained a man of no consequence.”

Accusing Mr Coyne of “pumping out lies” and a negative campaign, he writes: “It was vicious. It was horrible. It consisted almost entirely of personal attacks and smears against me.”

He also accuses his rival of being “not a good loser” and a “Trump-like crusade to overturn the election result”. But he also accuses Mr Watson of being involved in “this grubby attempt to dislodge me using underhand means”.

Mr Coyne is currently standing in the election to succeed Mr McCluskey against Steve Turner and Sharon Graham, with the result due in late August.