Chris Mason: All eyes on Downing Street - what does the PM say, and when?

For the third time in four years, we appear on the brink of a prime minister announcing their plan to leave office, not because they have lost a general election but because their own party has concluded they would be better off without them.

Many people within government and in the wider Labour Party expect Sir Keir Starmer to say just this, perhaps as soon as this morning.

Four years ago, and three prime ministers ago, Boris Johnson was running the country.

He was arguing defiantly he would carry on, but that defiance drained away as it became clear his capacity to viably govern was deserting him.

In early July 2022, I was standing in Downing Street as the lectern appeared and Johnson set out that, despite winning a big majority just a handful of years before, his own MPs were fed up of him and so reluctantly he was standing down.

Three months later, I was in the street again, as this time Liz Truss said she was leaving as she too had lost the support of her party.

And now here we are again, in all likelihood, with Sir Keir, less than two years after he won the general election to take over from Rishi Sunak.

His premiership, in the eyes of many on his own side, has been deflating for months.

And for months he has been trying to raise the bar as high as possible for any of his potential successors.

He made it very clear, via those close to him, that he wouldn't give up and he would stand in any leadership contest.

And as recently as last week he said Burnham's first priority if he won the Makerfield by election should be helping Labour win the Greater Manchester mayoralty he has now vacated, and that he would offer Burnham a seat in cabinet.

But Burnham's victory last week – and the scale of it – uncorked the bottle of Labour Party dissent that had been constrained by the by-election campaign.

Not only is Burnham now an MP, but he has proven, albeit in a region where he is very popular, that he can beat Reform UK.

And so, to many Labour MPs, frightened at Reform's popularity and recent electoral success, he is a better bet than Sir Keir.

It isn't plausible to simultaneously leave them in their roles for long and ignore their demands.

So option one, then, is sack them, replace them and stumble on, as the hot favourite to be your successor barrels down the West Coast Mainline to Westminster.

Burnham is expected in Westminster on Monday afternoon to be sworn in as an MP and a photo is then planned with Labour MPs.

But who knows what the prime minister may have said by then?

Option two for Sir Keir is seize what agency he retains and shape what happens next by setting a timetable for going.

Then there is the question of how long to hang around.

Some in the party see a contest as key to stress-testing Burnham and any others.

Some fret such a contest could appear inward-looking, gratuitous and drag on all summer.

The alternative, which appeals to some Labour MPs, is a process involving them, with hustings at Westminster – but not a full-blown vote involving trade unions and party members.

In that scenario a new PM could potentially be in place within a week or two – and sooner still if the momentum behind Burnham is overwhelming.