In years to come it will be possible to pinpoint the precise moment the Cabinet, Labour MPs and the wider Labour movement finally decided to cut Keir Starmer adrift. On Tuesday, Ed Miliband, a man who knows a thing or two about terminally doomed political leaderships, was being interviewed by GMB's Susanna Reid.

Reid was forensically taking apart the Prime Minister's case for appointing Peter Mandelson, and Miliband was gamely playing along, attempting to stick to No10's rigid and increasingly implausible Lines To Take on the issue.

And then, suddenly, Miliband stopped. He leaned forward in his chair and placed his hand on his chin. 'Yeah, it's a fair point,' he said, nodding. 'He shouldn't have been appointed. That is right.'

At the start of February, when Labour's Scottish leader Anas Sarwar became the first high-profile figure to call for Starmer to quit, the PM was saved by a concerted circling of the wagons by his ministers. After a week of conspicuous silence, they lined up one by one on social media to express their support.

This week, the opposite has happened. Quietly and discreetly, the wagons have been reversed out, and are disappearing into the sunset.

First there was Scottish Secretary Douglas Alexander, one of the Cabinet's most polished media performers, who on Monday broke the golden rule of not opening up speculation about a Prime Minister's political mortality. There were 'no certainties' about whether Starmer would lead his party into the general election, he conceded.

On Tuesday morning, Miliband finally threw in the towel. A few hours later, Foreign Secretary Yvette Cooper publicly distanced herself from Starmer's attempt to get his former communications director Matthew Doyle a plum diplomatic posting, despite his connection to a convicted paedophile, without her predecessor David Lammy's knowledge.

'I am, of course, extremely concerned at any suggestion that the permanent secretary or permanent under-secretary of the Foreign Office would be told not to inform the Foreign Secretary,' she said. 'As for the case that the honourable Member raised, I can confirm that it would also not have been an appropriate appointment.'

Then, yesterday, it was Work and Pensions Secretary Pat McFadden's turn to leave Starmer swinging. McFadden, who is known as the Cabinet's 'fire-blanket', conspicuously refused to endorse his boss's decision to sack Sir Olly Robbins. 'I think very highly of him,' he said cryptically.

All this was in public. In private, they queued up to distance themselves from Starmer's decision. According to a series of leaks from inside the Cabinet, David Lammy, Shabana Mahmood, Wes Streeting and Rachel Reeves all delivered thinly disguised criticism of the sacking.

Up until this week, some people were speculating that Starmer might be able to stagger on into 2027. But the events of the past 72 hours have decisively changed the mood within the Labour Party.

'It's moving', one Cabinet minister told me. 'People now recognise something is going to have to happen relatively quickly after the local elections. The discussions now are about finding a way for Keir to leave in a dignified way, and in a manner that isn't messy and splits the party.'

A second Cabinet minister agreed. 'Before Iran, the sense was that people would have to act after the local elections. Then Iran happened, and views shifted a bit. People started to think, 'I'm not sure how we can have a leadership contest with a war as the backdrop'. But now it's hardened up again. The feeling is basically that 'this is not going to end. Keir can't get out from under this. We need to draw a line'.'

Much of the debate around the latest Mandelson revelations has centred on whether Starmer knowingly misled parliament. But according to Labour sources, among senior figures in the Government, the Robbins debacle has seen a different set of perceptions forming. As one veteran told me: 'Up until this week people thought the main issue was structures inside No10, and the fact that Keir wasn't especially political. But now the basic view is 'the guy's useless. He just can't do this'. The respect for him has just collapsed. I've now got people sending me memes of him in a dunce's cap. It's gone.'

Another problem is that the slender hopes some MPs and Cabinet ministers were clinging to that Starmer could use the Iran war to reboot his premiership have finally been extinguished. As one minister said: 'It was always a long shot that we could turn things around via international affairs. But now there's no chance.

'Mandelson isn't coming up on the doorstep a lot. But nor is Trump or Iran. People aren't saying 'your guy's doing a great job over the Strait of Hormuz'. They're saying 'what are you doing about all those bloody illegal immigrants?'

The reality is that the so-called 'Love Actually Strategy' – a reference to the moment in the film that Hugh Grant's fictional prime minister finally confronts a bullish US President – is failing to cut through has finally been recognised inside No10. According to one minister, Downing Street's plan to get the latest tranche of Mandelson documents out of the way before the local elections, then have a clean relaunch based around the King's Speech, has now been junked.

'He's too weak now,' they told me. 'They know that there's enough in those documents and if they came out now, it would completely sink him. So they have had to shelve it. They're not bothering with a strategy any more. They're just trying to see if they can survive a day at a time.'

This is Starmer's political prognosis now. Not years, or even months. But days and weeks.

According to ministers, two scenarios are now under active consideration. One is a quick, clean removal, followed by the insertion of a 'caretaker leader'. This would involve – with the parliamentary Labour party's consent – a safe pair of hands imported from within the Cabinet. John Healey, Pat McFadden, Hilary Benn, Yvette Cooper and Bridget Phillipson are the names reportedly under consideration.

The other is what's been described to me as 'the Celebrity Death Match'. Starmer would announce his departure but stay on until Labour's September conference. That would enable the big campaigns of Wes Streeting, Angela Rayner and possibly Ed Miliband to properly mobilise, and also give time for King of the North Andy Burnham to secure a seat, and set out his stall. It would also give Labour members proper ownership of the process.

But either way, the decision has now been taken. As one normally loyal minister told me: 'It's just bleak now. I don't see any way of this stopping. This has been going on for seven and half months. And it's not going away. Mandelson keeps hanging around like a bad smell.'

This morning, the stench of political death is clinging to Keir Starmer. When the local elections – which will be an unmitigated disaster for Labour – are over, the undertakers will arrive.

'It's going to have to be a men-in-grey-suits operation [to remove Starmer],' one despondent minister told me. 'The great irony is that the perfect person to fulfil that role would have been Peter Mandelson.'

He is no longer available. But as Keir Starmer is about to find out, there are plenty of his colleagues around the Cabinet table who will be only too happy to take the Prince of Darkness's place.