'It is inevitable!' Boss of Labour's biggest trade union donor warns 'rudderless' Keir Starmer WILL resign

She also warned that the Government risks "sowing the seeds of its own destruction" if the Prime Minister's successor does not "depart from its current path".

But Ms Graham's piece, written in The Times, was clear: if a change at the top is made, the narrative must be different, otherwise it is simply pointless.

"It cannot just be a question of changing the leader or improving the sales pitch," she wrote.

"There is much agonising right now about whether, when and who should replace the Prime Minister. That is inevitable.

"But a new Labour leader with the same policies won’t cut it.

"Replacing Keir with Angela [Rayner], Andy [Burnham], Wes [Streeting] or Ed [Miliband] won’t persuade workers to return to the fold if they are just offering more of the same in a different voice."

Unite is Britain's second-largest trade union by membership, and was Labour's biggest donor this year, but is becoming increasingly disillusioned by policies.

It also became the only affiliated union not to endorse Labour's manifesto - this is only compounded by her description of Sir Keir's "failing leadership" and lack of belief in any other leader.

Unite's priorities were made clear: "Trade unions are there to fight for workers, not to side with politicians,” she said.

Ms Graham described Mr Miliband's net zero policies as economic "self-harm", and accused him of running a "jobless transition" away from fossil fuels.

Discussing last month's budget delivered by Rachel Reeves, the Unite general secretary said: "Opting for stealth taxes on workers instead of a wealth tax on the mega-rich was the wrong choice. Labour must stop being embarrassed to be the voice of workers."

Ms Graham has refused to rule out the possibility of Unite voting to end its affiliation to Labour next year, while the trade union declined to exercise its right to nominate either of the candidates in this year’s deputy Labour leadership election.

But it isn't just Ms Graham and Unite that could pose a huge problem to Sir Keir. Andrea Egan, a stark critic of the Prime Minister, was recently appointed chief at Unison, Britain's largest union.

"For too long it has been everyday people, workers and communities who have paid the price for crisis after crisis not of their making. In 2026 this must stop," Ms Graham wrote.

"The government needs to decide what it stands for and who it stands for. If we have to ask, it is not working.

"The party faithful can agonise about its failing leadership and a ‘night of the long knives’. But a new Labour leadership with the same policies simply won’t cut it.

"The doom loop cannot be broken with more austerity lite, no matter who is in Downing Street.

"Britain needs vision. We led the first Industrial Revolution and we are nowhere in the fourth. Rudderless."

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