Angela Rayner set to back Andy Burnham in plot to challenge Keir Starmer

Her intervention comes as mounting pressure builds on Sir Keir Starmer's allies within the party's ruling National Executive Committee to allow the Greater Manchester mayor to stand in the seat.

Mr Burnham has long been considered a potential challenger to the Prime Minister for the Labour leadership and a successful by-election victory would open a path back to Westminster.

He faces a tight deadline of 5pm on Saturday to inform Labour's NEC whether he intends to seek the nomination, with a decision on his eligibility expected on Sunday.

Under party rules, elected mayors must obtain permission before pursuing selection as parliamentary candidates, with sources suggesting supporters of the PM within the body will attempt to block his candidacy.

Several Labour sources told The Times the committee may object on grounds that Mr Burnham's candidacy would trigger a mayoral election in Greater Manchester, viewed as both financially draining and politically risky for the party.

The NEC could alternatively demand an all-women shortlist for the by-election, given male MPs currently outnumber their female colleagues.

A growing number of Labour MPs have publicly cautioned against any attempt to exclude Burnham from the selection process.

Jo White, who chairs the Red Wall Group of MPs, declared: "Let the north decide who their Labour candidate should be for the Gorton & Denton by-election. A London stitch-up will be a disaster for Labour."

Jon Trickett, a former shadow cabinet minister, expressed frustration, stating he was "fed up to the back teeth of London-based hacks who work in the dark imposing their ideas on the north".

Connor Naismith, MP for Crewe & Nantwich, added: "Gorton & Denton deserves the best possible choice of candidates. I agree with the prime minister that our attention should be on delivering for the public, not speculating about future leadership contests. Any decision made to limit the choice would be wrong."

One MP suggested deliberately excluding Burnham would "smack of desperation”, according to The Telegraph.

Trade union leaders have joined the chorus of voices opposing any move to block Mr Burnham.

Andrea Egan, general secretary of Unison, Britain's largest trade union, said: "I'm sure all trade unionists expect a democratic process for Gorton & Denton in which local party members decide who they want to represent them. We've seen enough control-freakery in the Labour Party and it has done our movement nothing but harm."

Steve Wright, of the Fire Brigades Union, went further, describing any potential block as "a democratic outrage".

Karl Turner, MP for East Hull, warned preventing the Greater Manchester from standing would make Sir Keir appear "frit," despite backing the PM.

"If he wants to stand, let him stand, and if there's a challenge on Keir Starmer I doubt it would happen I think Keir Starmer would see it off," he said.

"I'm not a Burnham supporter particularly, but what I am a supporter of is the Labour Party and the Labour Party winning. If Andy Burnham is our best candidate for that particular parliamentary seat, we should field him as the best candidate who is the most likely to win the seat."

Health Minister Stephen Kinnock sought to downplay the controversy, praising Mr Burnham as "an incredibly talented and effective leader" while noting the mayor had not yet indicated his intentions.

Deputy Prime Minister David Lammy urged caution, stating: "I don't know if Andy really wants to get back into Westminster politics. He may do, and there's much merit to him.

"But I would caution those often around folk that start to destabilise the ship, and say that we can have a new leader. No, we can't. Because immediately you'd be forced to have an election, premature probably."

The by-election was triggered after Andrew Gwynne formally resigned from the Commons, citing medical advice not to return to work.

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