Henry Zeffman: What does Keir Starmer do next after Andy Burnham's Makerfield win?

"Delusional."

"A bit deluded."

"Utterly deluded."

That's just a flavour of the messages I've been receiving from Labour MPs since Andy Burnham won the Makerfield by-election in the early hours of this morning.

To be clear, it's Sir Keir Starmer they're talking about: the man who not even two years ago brought Labour back to the promised land of government after 14 years away, and only five years after Labour's worst ever general election defeat.

These MPs span Labour factions, generations and ministerial ranks.

But they are united in their view that Sir Keir has entered the endgame.

To Sir Keir and his allies, it's those pushing for his exit who are deluded.

Their case this morning is that the spectacle of a governing party consumed by internal conflict is exactly what the public voted Labour to end.

"When the Tories lost the last election, Britons were most likely to see them as 'only interested in themselves', 'dishonest' and 'divided'," a document being circulated among the prime minister's supporters says.

"We cannot allow ourselves to be tarnished in the same way."

Yet for all Sir Keir's warnings against division, among many Labour MPs this morning there is actually a surprising amount of unity.

They are increasingly unified in the belief that Burnham's destination of Downing Street is assured, and that the only open question is the precise path for getting there.

The word being used more and more to describe the leadership contest which is now inevitable is "coronation".

In other words, MPs are saying they do not believe a leadership election will get to the stage where Labour Party members have their say.

Instead, they believe, Burnham will end up being the only candidate with the required backing of 81 MPs (and in this scenario he would get far more than that) and would therefore be elected by acclamation, as Gordon Brown was when he became Labour leader and prime minister in 2007.

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One complicating factor in any coronation could be Wes Streeting, who quit as health secretary in protest at Sir Keir's leadership last month.

Amid a measure of scepticism from some of his colleagues, Streeting has been adamant in recent days that he has the backing of 81 MPs required to trigger a contest.

We may find out within days, but he is likely to come under real pressure, including from some of his allies, to tuck in behind Burnham to enable a swift transition.

Of course the bigger complicating factor is Sir Keir, who reiterated this morning that he would be a candidate in a leadership election.

Under Labour's rules, Sir Keir does not need to hit the magic number of 81. As the incumbent, he is automatically placed on the ballot of party members.

In that sense, there can be no coronation. Sir Keir's commitment to press on guarantees a ballot of Labour members. He doesn't need the backing of any of his MPs, let alone 81 of them.

Yet that ignores the fact that he is prime minister as well as Labour leader. In our system, the prime minister can only lead if he commands the confidence of a majority of MPs.

It was the reality that he could no longer put together a functioning government which forced Boris Johnson's demise in 2022.

Does Sir Keir still have the backing of enough of his colleagues to lead the government? That's really the core question which we will find the answer to over the next few days.

Speaking to Labour Party staff this lunchtime, the prime minister said that a leadership election would "tear apart our party and our movement".

But if - when - one is triggered, Sir Keir needs to prove that it's not his own desire for survival which is more responsible for Labour's chaos.