Fresh backers flooding in today are said to include Steve Reed, a close ally of the outgoing Prime Minister as Sir Keir’s own inner circle threw in the towel.
His backing suggests even those closest to Sir Keir have accepted the leadership contest is now a foregone conclusion.
He is then poised to walk into No 10 as Prime Minister on July 20, inheriting a Government still reeling from weeks of Labour civil war sparked by Sir Keir’s shock resignation.
With time fast running out, no rival can now scrape together the 81 nominations needed to even mount a challenge, handing Mr Burnham the crown unopposed.
It marks a stunning ascent for the former Greater Manchester Mayor, who strolled back into Parliament only last month after storming to victory in the Makerfield by-election.
That win lit the fuse under Sir Keir’s premiership, with senior Labour figures rushing to fall into line behind the man dubbed the “King in the North” in the frantic weeks since.
Wes Streeting, once tipped as Mr Burnham’s fiercest rival, abandoned his own leadership dreams to back him, while Cabinet minister Darren Jones also bowed out of the race.
Mr Burnham’s allies say he will hit the ground running in Downing Street, pressing ahead immediately with plans to rip power out of Whitehall and hand it to the regions.
Despite being just days away from running the country, Mr Burnham has yet to face a public debate over his policies.
He has also not taken questions in Parliament, and is unlikely to do so as the House enters recess the day he becomes Prime Minister.
Kemi Badenoch, the Conservative Party leader, has urged the Speaker of the House to allow MPs to sit for a few extra days to question the new PM.
Sir Lindsay Hoyle, who was elected as a Labour MP before becoming Speaker, has yet to grant that request.