Any move to prevent Burnham from standing as an MP will now put Starmer at odds with senior party figures.
Former leader Ed Miliband heaped praise on Burnham, saying he would be a “massive asset in Parliament”.
Lucy Powell, the deputy Labour leader, and Sadiq Khan, the Mayor of London, also backed Burnham’s return to Westminster.
Health Secretary Wes Streeting, who is seen as a potential leadership candidate himself, said that anonymous briefings against Burnham were a “disgrace”, adding that he was “part of the team” and “should not be treated like that”.
It comes as the Prime Minister prepares to head to China this week, meaning he will be absent from Downing Street as his leadership is plunged into crisis and his party into a potential civil war.
Burnham – who is known as the “King of the North” – has made no secret of his ambitions to become Prime Minister.
In the run-up to the Labour Party conference last year he said many MPs had urged him to mount a leadership challenge against Starmer.
Andrew Gwynne, a suspended Labour MP, confirmed this week that he would step down from his north-west England seat of Gorton and Denton, opening the door for Burnham to seek a return to Westminster via a byelection.
Announcing his intention to run, the Manchester mayor promised that he would not “undermine” the Prime Minister’s leadership.
As a current Labour mayor, Burnham must seek the “express permission” of the NEC to stand.
The 10-member committee is expected to announce its decision within the next 24 hours.
There was mounting support for Burnham’s return to Westminster, with left-wing politicians and friends of Burnham insisting he should be allowed to become an MP.
‘He will be a massive asset’
Speaking at the Fabian society conference, Miliband, the Energy Secretary, said: “I think Andy has done an outstanding job in Greater Manchester. I think he will be a massive asset in Parliament, so my view on this is that I very much hope that the local party will have the option of picking Andy as a candidate.”
Asked whether it raised questions about Starmer’s leadership, he added: “Not in my view. I think it’s really important Keir needs the best team serving under him, and the Labour Party needs to look outwards and not inwards – and it needs to be loyal to Keir and it needs to get on with the job of serving the country.”
Speaking at the same event, Khan said he would “knock some doors for him” if his Greater Manchester counterpart decided to stand in an upcoming byelection.
He added: “If Andy Burnham wants to be a member of Parliament, Andy Burnham should be a member of Parliament”.
Powell told the socialist gathering: “I think it’s going to be maybe the most consequential byelection of this Parliament. And in that context, I would want to see the very best candidate to help Labour win in that very difficult byelection.
“I want the best candidate to come forward and someone who will be able to move forward to help us win.”
Shortly after he announced his candidacy Mainstream, a newly formed soft-left group endorsed by Burnham, launched a petition directed to Home Secretary Shabana Mahmood, the NEC chairman, calling for a “fair, democratic and open selection” in Gorton and Denton.
In an email sent to members, Mainstream said: “Local party members, and the people of Gorton and Denton, must have the opportunity to select and elect the strongest possible Labour candidate to take on Reform – with no qualified candidates excluded”.
‘It’s worrying. We must unite’
Several Labour backbenchers, including Karl Turner and Nadia Whittome, piled pressure on Starmer’s supporters not to block the northern mayor’s return to Westminster.
In a series of social media posts, Labour MPs warned of a rupture in the party if Burnham was blocked from standing.
“This idea of attempting to block Andy Burnham from standing is going to cause the most raving row by the reaction of some Labour MPs,” Turner said. “It’s very worrying. We must unite.”
Whittome said: “Sorry but if the most unpopular Labour government in history intervenes to block our only senior Labour politician with a net positive public approval rating – in doing so risking handing victory to a far-right party – that’s putting petty factionalism before the country.
“The last person at Labour HQ had better turn the lights off on their way out.”
Ian Byrne, a Labour MP, said the party “should be begging” Burnham to stand, adding: “Keeping Reform out of Gorton and Denton is imperative”.
‘Bring on the super sub’
Chris Webb, another Labour backbencher, added: “Blocking Andy Burnham from Gorton and Denton will have serious lasting consequences. When you’re 2–0 down at half time, you bring on the super sub to help the team win.”
Kim Johnson, an MP on the left of the party, said: “Blocking Andy to satisfy factional interests would be a serious mistake and would have long-lasting consequences”.
Cat Smith, a former frontbencher, told the BBC on Saturday that party members – not Labour HQ – should decide who stood in the forthcoming byelection.
It would be “disappointing” if No 10 blocked “strong candidates” from standing, she concluded.
But the Prime Minister’s allies on the NEC have been ramping up their rhetoric about the Greater Manchester Mayor’s unsuitability to run as an MP.
“You can’t really find anyone on the NEC that thinks it is a great idea for him to stand,” one said. “There are innumerable reasons why it is a really terrible idea.”
They explained that the Labour Party was “absolutely skint” at the moment and it would be costly to run a mayoral election campaign, should Burnham vacate the post to run as an MP.
However, the MP acknowledged that blocking Burnham’s return could come at a hefty cost for Starmer.
“Nothing that you do in politics comes without a price,” a source said. “If we did this, what signals does it send? Is the publicity worse?”
A Labour insider told the Telegraph that there is no way Burnham will be approved by the NEC as the “numbers aren’t there”.
Burnham issued a lengthy statement today explaining why he wanted to return to Westminster after nine years as mayor.
He said it had been a “difficult decision” but he had given “careful thought” to what was in the best interests of the Labour Party and of Manchester.
Praising Starmer, Burnham said: “The progress already made on rail renationalisation, bus re-regulation, the housing crisis and devolution is truly impressive. My role in returning would be to use my experience to help it go further and faster, as well as communicate the difference it is making.
“I would be there to support the work of the Government, not undermine it, and I have passed on this assurance to the Prime Minister.”
Burnham said he would run a “hopeful and unifying campaign” that would focus on “being honest about the alienation people feel from politics”.
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