Could this be the beginning of the end for Starmer?

This is a very serious moment for Sir Keir Starmer.

Everything the prime minister has said and done today has reflected the fact that he and his team understand the gravity of his situation.

It would have felt odd and jarring had Sir Keir simply delivered the speech that he had been planning to deliver this morning about more funding to inspire pride in neighbourhoods.

The prime minister instead delivered an apology specifically to the victims of Jeffrey Epstein for having "believed Mandelson's lies" and appointed him to the UK's top diplomatic posting.

Arguably more telling was the moment when, under intense questioning from journalists, Sir Keir conceded that he understood the "anger and frustration of Labour MPs".

It is Labour MPs who will determine the prime minister's fate and they are more furious than they have ever been during his leadership right now.

Labour MP Rachael Maskell, who has been a thorn in Sir Keir's side over welfare cuts, has said she thinks his position as prime minister is "untenable" and it is "inevitable" that he will have will have to stand down.

She told BBC Radio York: "I don't believe he's got a choice.

"He kept from the House [of Commons] for several months the fact that he knew of the associations between Peter Mandelson and Jeffrey Epstein at the point of appointment."

This was a "dishonourable thing to do" to MPs, as well as Epstein's victims, she added.

Another Labour MP, who did not want to be named, told BBC Radio 5 Live's Matt Chorley: "I have to say, I don't see how he can carry on."

"It's been terminal for months," added another, "but the patient has now stopped responding to treatment."

A Labour minister told Matt: "The only thing that is certain is that the government is not in control of this and so it could go anywhere."

Others Matt spoke to were more supportive of the PM, with Rugby MP John Slinger saying: "Calm heads need to prevail. The PM has done the right thing here."

Steve Witherden told BBC Wales: "At the very least, the prime minister's chief of staff, Mr [Morgan] McSweeney, clearly has to answer why he pushed for Mandelson's appointment despite details of his relationship with Epstein being publicly known."

However, it is telling that for all the off the record anger, few MPs are calling for the prime minister to go in public.

The small handful who have so far are MPs who have never much supported his leadership.

In fact, there are not even many MPs willing to call publicly for McSweeney to be sacked, something Sir Keir has resisted doing.

That suggests that while the prime minister is weaker than ever, a challenge to his leadership is far from imminent.

"Realistically I don't think something can happen until post-Gorton", one MP said, referring to the Gorton and Denton by-election on 26 February. "And then it might be too close to May."

A former minister said: "I still think it is very unlikely anybody moves before May. But it is all moving very fast."

They added: "I don't see how the PM recovers from the utter lapse in judgment in appointing Mandelson."

A current minister said: "He's done. It's just a question of when."