Shortly after 10 o'clock this morning Andy Burnham finally confirmed what had been obvious for days - that he would stand to be the next Labour leader and therefore the UK's next prime minister.

The new MP for Makerfield, while on a train headed from Manchester to London, took to social media to say: "I will put myself forward as part of this process."

By the time his train arrived at London Euston station, his most likely leadership rival Wes Streeting had already thrown in the towel and backed Burnham for the top job.

At that moment the chance of a full-blown leadership contest, which Sir Keir Starmer appeared to advocate in his resignation speech, began to look vanishingly thin.

Streeting had previously spoken of the need for a "battle of ideas" but now said it was better not to "spend the summer exaggerating small differences".

The move surprised many of his backers.

In the moments before Streeting's announcement, one source close to the former health secretary said "lots of colleagues" were still urging him to run against Burnham.

Another supporter said there needed to be a contest because "the lack of clarity [from Burnham] is concerning".

Streeting acknowledged that he and Burnham had "spoken at length" since the latter's by-election victory, but denied there'd been a deal in which he was offered a job.

Nevertheless, it's now widely expected across the Labour Party that a Burnham premiership would see Wes Streeting handed a senior cabinet position.

Angela Rayner, the former deputy prime minister who had also been talked up as a potential leadership candidate, said Labour "must now redouble our efforts to deliver for working people".

She chose not to explicitly endorse Burnham, but is not thought to be preparing a leadership bid herself.

Through the course of the Makerfield campaign there was a growing feeling among Starmer loyalists that Andy Burnham was struggling to stand up to scrutiny.

A BBC Newsnight interview in which he declined to name the government's fiscal rules, which he had pledged to stick to, only crystallised those thoughts.

He refused to answer many questions about national policy on the grounds that he was merely running to be the MP for Makerfield.

Some MPs loyal to Sir Keir had discussed the idea of putting up a candidate from their own side if there was no other challenger, as a way of testing Burnham's policy platform.

The prime minister's chief secretary Darren Jones had been talked up by some as one potential contender.

But if there was any doubt that the leadership election was over before it had begun, the whoops and cheers with which around 200 Labour MPs greeted Andy Burnham as he arrived for a group photo in Parliament ought to have expunged it.

Holding such a photo is standard practice with any by-election winner.

But the crackle of anticipation in Westminster Hall made abundantly clear that this was anything but standard. The mood was beyond celebratory.

It was impossible to separate the photo with a by-election winner form the gathering to anoint a new leader which this also seemed to become, especially considering some of the MPs present.

Rachel Reeves, the chancellor, and Jonathan Reynolds, the chief whip were both front and centre – neither of them having been there to watch Starmer resign about six hours before.

Jones himself was also present, alongside other major Starmer allies including Steve Reed, the housing secretary, and Nick Thomas-Symonds, responsible for EU relations.

As Boris Johnson put it when he was ousted by Conservative MPs in 2022: when the herd moves, it moves.

After the formal photo, Burnham got out his phone and took a selfie with the throng, straining to fit them all into one shot.

Those stood behind Burnham included MPs from all sides of the Labour Party.

Along with Streeting and Jones, both seen as being on the right of the party, major players from the left, who had been central to Jeremy Corbyn's Labour project, also turned up to cheer on Burnham.

One government aide still loyal to Starmer remarked that they were glad they wouldn't have to figure out how to keep all sides happy.

A rump of backbenchers who believe the party was wrong to oust Starmer and a separate group who think Streeting would have made a better successor could still make it difficult to unite a fractious party.

There's disagreement among those close to Burnham about how quickly to move.

His team had made clear over the weekend that their preference was for a changeover of power in September.

But if no contest presents itself, it looks likely that Burnham could walk through the door of Number 10 as soon as 16 July.

One source who has been working on policy for Burnham for a number of months described their preparations as "not very ready" and said they could still try to keep Starmer in place until the autumn, even if there is no leadership contest.

They suggested that prospective cabinet ministers and advisers could be picked before then and put through "accession talks" in August to better prepare them for government.

But that's not a view held among the majority of Burnham's inner circle.

One source said "the timetable is the timetable, we don't have a choice, he becomes PM in July if he's the only candidate".

The timetable for a leadership contest is decided by Labour's ruling National Executive Committee, but if there is no contest it could well be down to Starmer himself.

One ally of the prime minister's said they didn't think he would be willing to hang around just to give Burnham more time to prepare.

Policy work has been under way for weeks in areas such as housing and transport.

Some advisers have been looking at ways in which a Burnham-led government may be able to borrow more in some areas.

But Burnham's allies accept there's been less developed thinking when it comes to defence, energy and welfare.

At the end of another tumultuous day in Westminster it looks all but certain that it will now fall to him to fit all of this together into a cohesive government and functional political project.