Last week, a Tory MP and his wife were attending a swish Westminster party when they spotted a figure standing alone and apparently lost in his thoughts. It was Keir Starmer.

‘I felt sorry for him,’ the MP told me. ‘There was no one near him. So I went over and said, “Hello Prime Minister, I hope you’re bearing up”.’ Starmer looked at him for a moment, then responded wistfully: ‘You still call me Prime Minister. Not many people bother to do that any more.’

In just over three weeks, no one will. In the immediate aftermath of his emotional resignation speech in Downing Street, most people thought Starmer had agreed to a lengthy and orderly transition of power, with the final handover to his presumed successor, Andy Burnham, coming at the start of September.

But they had misinterpreted his words and his intentions. His agreement to stay in post for another three months was conditional on there being a formal Labour leadership contest. And Starmer knew full well that this was something Burnham was keen to avoid.

As one Starmer ally told me: ‘He’s not going to sit in No 10 twiddling his thumbs while Burnham gets his robes measured for a coronation. If the party wants him to keep the ship steady through an election, fine. But if not, he won’t be hanging around.’

Within Labour’s deeply divided ranks, there are two interpretations of Starmer’s decision. To his supporters – and even several of his erstwhile opponents – this is the legitimate response of a man who has just been brutally driven from office and can’t be expected to have his assassin’s best interests at heart.

As one Starmer ally told me: ‘Andy’s team were desperate for him to wait until September to hand over. They wanted time to properly prepare. Well, tough. You asked for this. You’ve spent the past month telling everyone, “That’s politics: You don’t get to choose when you enter or leave the stage”. Fine. Stop crying, suck it up, and get on with it.’

Within Team Burnham, there is a view that Starmer is guilty of deliberate sabotage. ‘He’s tried to kneecap us before we’ve even started,’ one told me last night. ‘He could have waited till the [September’s party] conference, and we could have had a respectful handover. But he’s decided just to walk away.’

This, in truth, was always Burnham’s greatest fear. Over the past few months, as his team has plotted his tortuous path to the premiership, there was one message they pushed consistently.

‘We need space to prepare,’ a senior ally told me in the hours after the Gorton and Denton by-election in February. ‘Keir is finished now. But Andy needs time to get everything in place. We need to get a proper team together. We need to get a clear policy programme. And he needs to get himself into the “headspace” where he is ready to be Prime Minister. That can’t happen overnight.’

Now, thanks to Starmer’s decision to expedite his own departure, it will all have to happen overnight.

The first warning of the PM’s intentions began to reach Burnham and his allies on Friday evening. Although there was no direct communication between the two men – Starmer had become ‘volcanic’ at the mention of Burnham or Wes Streeting’s names, according to one friend – tentative channels had been opened by Labour Party deputy leader Lucy Powell and a couple of Cabinet ministers.

Initially, it seemed that Starmer could be nudged into line behind ‘The September Strategy’, which would have involved a handover on the eve of, or even during, Labour conference. But Sir Keir’s mood hardened on Friday afternoon after a ring-round of supposedly loyal ministers. A series of them told him they could no longer support him and believed he would have to step down.

‘That shook him,’ one minister told me, ‘and it made him furious. He saw it as the ultimate betrayal.’

Later that evening, Burnham’s senior aides began to get word that Starmer was contemplating walking away. ‘I think he’s going to jack it in,’ one told me with mounting concern. ‘I don’t think he’s going to go for an orderly handover. I’m hearing he’s decided, “Screw the lot of you, I’m out of here”.’

These fears were heightened when No 10 consistently rebuffed Burnham’s attempts to organise a face-to-face meeting with Starmer to discuss the terms of any handover. ‘It’s not happening,’ one Burnham supporter told me. ‘Starmer isn’t going for it.’

When the PM appeared at the Downing Street lectern, Team Burnham were still not sure what he was planning to announce. Initially, their spirits rose when they heard Starmer mention a September handover. On the Burnham campaign WhatsApp group, a message flashed: ‘Prepare for transition on September 1.’

But then the reality of what Starmer was really planning finally began to sink in. As the news of the PM’s accelerated departure broke, one Burnham ally exclaimed: ‘F***! We don’t want that.’ Another tried to put a brave face on things, insisting: ‘It gives us a bit of time to get organised. Then we’re into summer recess, which gives us more space to work on things in the run-up to conference. We can make it work.’

The reality is that Starmer’s cynical – if understandable – ploy to pull the rug from Burnham before his usurper has even set a foot inside Downing Street is going to create massive problems for the self-styled ‘King of the North’. A look at the successful modern Prime Ministers – Thatcher, Blair, Cameron – reveals one thing in common. Each had years to prepare themselves for their elevation to the highest office in the land.

Burnham will have 21 days. He has no mandate. His programme for government, if there is one, has not been stress-tested. He will not have time to pull together a proper and detailed strategy for his first 100 days in office. He will have no settled team. And, crucially, he will have had no time properly to prepare himself mentally for his accession to the most stressful and demanding office in the land.

Yesterday I spoke to a former aide to Liz Truss, the 49-day PM. He said: ‘She had a couple of months to prepare, because it was clear from relatively early on that she was going to beat Rishi in the leadership election. And she also had a settled view of what she wanted to do in office. But nothing can really prepare you for that first impact with the system. And perhaps even more significantly, the psychological impact of becoming a national leader.’

Streeting’s announcement that he was shelving his own leadership bid, minutes after Starmer had walked back into No 10, meant the prospect of a Labour leadership contest was removed. Although, in fairness, that was a problem of Burnham’s own making.

But I can reveal that the official line from Streeting is that he opted not to stand because of assurances he had received during a weekend of negotiations between himself and Burnham.

But the truth was that a bombshell spreadsheet containing the names of MPs who had pledged to support any leadership challenge by Streeting had been leaked to Burnham’s team. This has meant that for the past fortnight, Louise Haigh – Burnham’s wily campaign manager – had been methodically peeling off Streeting’s backers. ‘By the end, Wes wasn’t even close to having the 80 names [needed to trigger a leadership contest],’ one MP told me.

Yesterday, Burnham’s colleagues were upbeat as they gathered on the steps of Parliament’s Westminster Hall to be photographed with their next leader. But behind the smiles, several Cabinet ministers were articulating concerns over the impossibly tight transition timetable. ‘He’s not going to be close to ready,’ one told me. ‘For example, he can’t even begin to do any serious civil service prep until he’s confirmed as leader.’

Said another: ‘[His allies] were trying hard to get a longer timetable. And there was a stage where it looked like Keir wanted that, too. But that’s all gone now. And it’s problematic.’

That may prove to be one of the great understatements in modern political history. In his resignation speech, Starmer made great play of his unwavering commitment to the national interest. ‘Every decision I’ve taken has been about putting the country I love first,’ he insisted. But as with so many of Sir Keir’s statements, this wasn’t true. By refusing to stay in office to oversee a proper, orderly transition, he has instead attempted to handicap his successor.

The Westminster game is a vicious one. And having been cast aside by his party and – in his own eyes – treacherous MPs, Starmer clearly feels well within his rights to pay them back in kind. The problem is, if Burnham implodes, it is not Sir Keir but the British people who will end up paying the heaviest price.

But our outgoing Prime Minister doesn’t really care about that, and nor do his small but committed posse of vengeful former allies. Soon after Burnham was sworn in as a new MP, I bumped into one of them transiting Westminster’s labyrinthine corridors. How long would it take them before they started manoeuvring against Burnham, I joked.

‘Everything can wait,’ they said, with a glint in the eye.

The King is dead. The plotting against the new King is already under way.