Three things that could lead to Starmer's downfall - or save him

This has been Sir Keir Starmer's worst week as prime minister.

There is a persuasive argument that it will be remembered as the moment his downfall became inevitable.

This a view shared by some Labour MPs, but that does not mean his departure from Number 10 will definitely happen in the near future.

Westminster has seen plenty of leadership crises over the past decade. Sir Keir has faced questions over his leadership for months.

But when a leader is under pressure, there are key questions worth asking:

Here are three moments that could prove dangerous for the prime minister.

Firstly, the publication of documents connected to the appointment of Peter Mandelson as the UK's ambassador to the US.

It is not clear when these will emerge. It could be some time.

Sir Keir believes this material will prove that Mandelson lied throughout the vetting process about his links to Epstein.

But will they back up the prime minister's claims in black and white?

The breadth of documents ministers have been told to publish is huge - including communications between Mandelson, ministers and special advisors.

That could throw up more difficult questions and embarrassing revelations.

If those revelations relate to Number 10 - that would put more pressure on the prime minister.

26 February - Gorton and Denton by-election

Labour won this seat comfortably in 2024 - but they face a real struggle to hold on to it this time.

There are some in the party who fear they could even finish third - behind both the Greens and Reform UK.

If the result is a disaster for Labour, it would be tangible example of recent troubles having an impact at the ballot box. That would inevitably spook the party's MPs.

7 May - Elections in Scotland, Wales and English councils

May's elections have long been seen as a moment that would tip things over the edge for the prime minister.

There is a widespread expectation Labour will lose power in Wales for the first time since devolution, potentially finishing third behind Plaid Cymru and Reform.

Hopes of taking power in Scotland seem to fading, with third place, behind the SNP and Reform seen as real possibility.

And Labour fear a thumping in local council elections in England.

If the prime minister is still in post in May, this could be the most dangerous moment yet - when the extent of Labour's electoral collapse becomes clear.

A few important caveats...

There is not an easy mechanism for Labour MPs to remove a leader cleanly and quickly.

An MP could mount a leadership challenge - but that would take weeks, if the prime minister decides to fight on.

There are conversations about alternative ways of forcing the prime minister out - such as a confidence vote among Labour MPs or a coordinated message from senior figures.

But these methods are not guaranteed to work.

But at the moment, it's not clear who would lead that charge.

The next big caveat: there is not an obvious successor to Sir Keir at the moment. In fact, the main names which have been linked with the job face significant problems.

Health Secretary Wes Streeting has longstanding links to Peter Mandelson.

If the Mandelson crisis brought down Sir Keir, would Labour MPs really turn to someone so close to him?

Former Deputy Prime Minister Angela Rayner is favoured by some on the left.

But she still being investigated by HM Revenue and Customs over tax on her second home, with no sign of a final resolution being reached yet.

Other names mentioned as "safe pairs of hands" include defence secretary John Healey and armed forces minister Al Carns.

There has even been some talk on the so-called soft left of the party about Energy Secretary Ed Miliband returning to the leadership, however unlikely that may seem.

Others may look to Home Secretary Shabana Mahmood.

But the field could be quite open - and that makes many MPs nervous that they might not get the leader they want.

Would the centre of the Labour Party bring down Starmer if they feared they might end up with a figure from the left?

Would the left fear a swift process that could install someone they dislike?

It's worth remembering international events too.

Some Labour MPs would also be wary of sparking a leadership election - with an uncertain outcome - while the war in Ukraine rages on and President Trump continues with his unpredictable approach to diplomacy.

There is no doubt Labour MPs are angry - and many are willing to talk at length about the prime minister's future.

They may well return to Westminster next week even angrier after judging the mood back in their constituencies.

But leadership crises can be drawn out affairs. Sir Keir Starmer may be in Number 10 for a while yet.