Wes Streeting has quit as Health Secretary, telling the Prime Minister he has lost confidence in his ability to lead the Labour Party.
In a damning resignation letter, the man eying up the top job accused Sir Keir Starmer of a "vacuum" in vision and allowing the Government to "drift".
He said: "Leaders take responsibility, but too often that has meant other people falling on their swords.
"You also need to listen to your colleagues, including backbenchers, and the heavy-handed approach to dissenting voices diminishes our politics."
It follows a day of mounting speculation the Ilford North MP was eyeing up a bid to boot the PM out of No10.
Nevertheless, a Downing Street spokesman repeatedly said that Sir Keir had his "full confidence" in his Cabinet minister.
In his resignation letter, Mr Streeting said it was "now clear" Sir Keir would not lead Labour into the next General Election.
He said his successes in improving the NHS were "all good reasons for me to remain in post".
Mr Streeting resigned from the frontbench this afternoon
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However, the MP continued: "As you know from our conversation earlier this week, having lost confidence in your leadership, I have concluded that it would be dishonourable and unprincipled to do so."
"It is now clear that you will not lead the Labour Party into the next General Election and that Labour MPs and Labour unions want the debate about what comes next to be a battle of ideas, not of personalities or petty factionalism."
Mr Streeting declared that the upcoming leadership contest to replace the PM must be "broad" and Labour must boast the "best possible field of candidates".
His letter also cited Labour's "unprecedented" local elections results last Friday, when the party lost its 27-year grip on the Senedd as well as swathes of support across England and Scotland.
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Mr Streeting accused Nigel Farage of posing an 'existential threat' to Britain
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The former Health Secretary explained that Whitehall's widespread "unpopularity", the rising threat of Nigel Farage's Reform UK, as well as policy "mistakes" were all reasons for his departure.
He said: "Where we need vision, we have a vacuum. Where we need direction, we have drift. This was underscored by your speech on Monday."
Mr Streeting revealed that he informed his boss he had "lost confidence in his leadership" during their private showdown on Wednesday morning.
But the latest to join the Labour backbenches also took the opportunity to accuse Mr Farage and his party of posing an "existential threat" to Britain's future and "representing a dangerous English nationalism".
"This represents both an existential threat to the future integrity of the United Kingdom, but Reform UK also represent a threat to the values and ideals that have made this country great," he wrote.
Progressives across the UK "understand this threat and our responsibility to confront it", he said.
"But they are increasingly losing faith that the Labour Party is capable of rising to our historic responsibility of defeating racism," he added.
Despite his rumoured ambitions, Mr Streeting stopped short of triggering a formal leadership contest.
Nevertheless, his resignation will continue to fan the flames of a possible leadership contest.
A candidate vying for the top job would require the backing of 80 MPs to launch a formal leadership bid.
This morning, it was doubted that Mr Streeting had the numbers to draw up the battlelines so explicitly.
Angela Rayner, Ed Miliband and Andy Burnham are just a few of the names believed to be gearing up to throw their hat in the ring.
But Sir Keir is seemingly unwilling to go down without a fight. Allies close to the Prime Minister have vowed he will stay on in his role and fight any leadership challenge thrown his way.