Sir Keir Starmer is under pressure over the “anti-democratic” decision to block a left-leaning Labour mayor from running in the North East.

Unite, the party’s biggest union donor, warned of “serious consequences” over the “major mistake” of barring Jamie Driscoll’s candidacy.

Greater Manchester mayor Andy Burnham and his counterpart in the Liverpool City Region, Steve Rotherham, said the move does not seem “democratic, transparent and fair”.

Mr Driscoll, the serving North of Tyne mayor who is described as the “last Corbynista in power”, was excluded from the longlist to run in the new expanded authority.

A senior Labour source linked the decision to Mr Driscoll sharing a panel with filmmaker Ken Loach, who was expelled from the party amid efforts to root out antisemitism from the party.

But figures on the left of the party have blamed “factionalism” under Sir Keir’s leadership.

Mr Driscoll said preventing Labour members from having a say over whether he should represent the North East was “frankly shocking” as he defended his “fantastic” track record.

He told Sky’s Sophy Ridge On Sunday show: “In a two-party system, if you’re going to ban people who are promoting socialist views from participating in that, that is really quite anti-democratic.”

The mayor defended discussing Loach’s films, some of which have been produced in the North East, despite the director downplaying the issue of antisemitism in the party.

“My understanding is he’s made all sorts of clarifications that he’s not a Holocaust denier and I think he wrote a letter to the New York Times explicitly saying that the Holocaust was a real event, which of course it was,” he said.

Loach, the director of socially critical films including I, Daniel Blake, was expelled from Labour in 2021 during what he called at the time a “purge” of Jeremy Corbyn’s allies.

Unite general secretary Sharon Graham warned that if Labour “remains intent on only selecting nodding heads” then it will “continue to make serious policy mistakes”.

“These actions by Labour are a major mistake and have serious consequences,” she added.

Mr Burnham and Mr Rotherham expressed their “concern” in a joint letter to Johanna Baxter, chairwoman of Labour’s National Executive Committee (NEC).

“Whilst we appreciate the NEC’s important role in upholding standards within the party, and rooting out any form of antisemitism, racism and discrimination, it also has a responsibility to ensure decisions are democratic, transparent and fair,” they wrote.

“To exclude a sitting mayor from a selection process with no right of appeal appears to us to be none of those things.”

They said Mr Driscoll should be entitled to an appeal process and “deserves to be treated with more respect than he has so far been shown”.

Shadow business secretary Jonathan Reynolds “strongly” disagreed with claims that Sir Keir is trying to purge the left of the party.

When we said we’d have zero-tolerance for antisemitism, when we said we would tear it out from its roots, we were serious about that

He told Sky: “Specifically in a case where somebody shares a platform with someone who themselves has been expelled from the Labour Party for their views on antisemitism, for opposing the tough action that needed to happen, that would preclude them from being a Labour candidate going forward.

“Because when we said we’d have zero-tolerance for antisemitism, when we said we would tear it out from its roots, we were serious about that.”

Loach was criticised for claiming antisemitism is being used to purge Labour of politicians on the left as he defended Mr Driscoll.

He told the PA news agency that keeping the mayor off the longlist published on Friday because they shared a platform was the “lamest excuse I’ve ever heard”.

The UK’s equalities watchdog found that Labour was responsible for unlawful acts of harassment and discrimination while Mr Corbyn was party leader, as he struggled to tackle antisemitism.