It still feels improbable that the British prime minister, Keir Starmer, will face a formal challenge even if, as assumed, his Labour party performs disastrously in next month’s local elections. But for many of his MPs, the latest revelations about Peter Mandelson have emphasised that the question is simply one of when, not if.

“It does seems incredible that he didn’t know, but the problem is that it’s quite possible as well,” was the summary of one backbencher, in response to No 10’s insistence that no one had told the prime minister that his pick to be the UK’s ambassador to Washington had failed his security vetting.

Some MPs believe the Mandelson vetting fiasco could be terminally damaging for a prime minister who, as one said, had painted himself as “whiter than white”. “I can’t see how he survives this,” one said. “I just don’t think it’s feasible for him to say he didn’t know anything. I’m angry and really sad.”

This, however, seems to be a minority view. For weeks there has been a growing consensus within the parliamentary Labour party (PLP) that terrible results on 7 May for elections to the Scottish and Welsh parliaments and English councils will not be the end point for the prime minister, for several interconnected reasons.

A fairly obvious one is that changing leaders during a war is not a reassuring look. Since the US-Israeli attacks on Iran began at the end of February, the UK has been a semi-participant and observer, however unwilling, and is braced for the economic fallout.

Almost as significant was the moment earlier that month when Anas Sarwar, the Scottish Labour leader, called on Starmer to quit. It came to nothing, but Labour MPs were granted a glimpse into the abyss and many seemed to decide that following the Conservatives down the regicide route had many drawbacks.

Finally, none of the probable challengers, such as the health secretary, Wes Streeting, or the former deputy leader Angela Rayner, appear ready to act – or in the case of the Manchester mayor, Andy Burnham, are even in parliament to do so.

None of this is to underplay the sense of sheer despair among many Labour MPs at the ability of Starmer and his allies to repeatedly walk the ball into their own net. On top of the sense of haplessness and drift are worries about how Starmer’s No 10 has a tendency to sacrifice others – in this latest case Olly Robbins and the Foreign Office – when things go wrong.

One MP described Starmer as “almost like a Bond villain – he always manages to escape the scene before the explosion, but he’s running out of people to push under the bus”. They went on to list those they felt had already been sacrificed: “Ethnic minorities, colleagues, the soft left, even Morgan McSweeney.”

While being prime minister can be a brutal business, there is a growing sense that Starmer’s willingness to jettison people to protect himself, even those who have shown him only loyalty, is becoming a liability. Nor is there much confidence that No 10’s rapid announcement of yet another internal inquiry, and the promise of a prime ministerial explanation to the Commons on Monday afternoon, will do much for the party’s mood.

“Strategy is all Starmer and his allies cared about,” one MP said. “Monday will be a shit show given he can’t even express feeling.”

While an imminent challenge still feels unlikely, increasing numbers of MPs are thinking about what will happen when the moment comes, a consideration for parts of the 2024 intake as well as elements of the older guard.

As one MP put it: “There’s a point at which the PLP always realises that loyalty is akin to complicity. It’s obvious the body count will rack up at the local elections.”

Some hope Starmer himself will come to a realisation, with one MP questioning whether the prime minister may accept that “he was just a caretaker – he’s had his fun and it’s time to enjoy other things in his life”.

When this crunch point may arrive depends on a number of factors, not least what happens in the Middle East. Many of Starmer’s ministers, as well as many MPs, agree he has charted a tricky course for the UK during the conflict relatively well, and are happy to see him respond more robustly to Donald Trump’s White House.

“The changed response to Trump hasn’t been planned – it’s just a natural response to events,” one cabinet minister said. “There comes a point where Trump says so many rude things that it would be almost strange to not hit back at least a bit.”

If the war ends relatively soon and with limited economic damage, one catalyst could be Labour’s annual conference in late September, when the party’s scattered geographical and ideological groups will come together to mourn the May results and look ahead to a general election that will, by then, be less than three years away.

It is equally possible that Starmer will limp on a bit longer, until another dismal set of local election results proves the catalyst, or as with the demise of Boris Johnson, a seemingly quite low-level scandal finally tips the scales into outright rebellion.

The blunt truth is that – for all ministers may argue about the dangers of panicking amid a UK political landscape that is unprecedented in its volatility and atomisation – as things stand, Starmer is leading his party off an electoral cliff.

The most recent YouGov polling put Labour in fourth place, on 17%, a rating that has not crept above 20% for six months. Nearly three-quarters of voters think Starmer is performing badly as prime minister.

“There’s not a lot of love for us on the doorsteps and there’s particularly not a lot of love for the prime minister,” one MP put it wearily. “This is more than just midterm blues. And everyone knows it.”