The Cabinet minister let out a sigh of relief. 'All the leadership camps have signed up to a September timetable. Most of the Cabinet is on board. The parliamentary party are moving behind Andy. The war's finally over,' they told me.

The war is not over. And the reason it isn't is that Keir Starmer and his increasingly fanatical coterie of allies refuse to bow to the inevitable and lay down their arms.

At 5pm yesterday, it was suddenly announced that Andy Burnham had been cleared to run in the upcoming by-election in Makerfield after an unprecedentedly swift and covert consultation of Labour's ruling NEC committee.

The reason for the subterfuge was revealed to me by a Labour minister: 'We had to move fast and in secret to stop Starmer's people wrecking the whole thing.'

Indeed, that's what No 10 had been trying to do for the past 24 hours.

On Thursday afternoon, Josh Simons, the MP for Makerfield, stunned Westminster by announcing he was stepping aside to allow Andy Burnham to return to Parliament. 

A few minutes later, Burnham issued a short statement in which he confirmed his intention to seek selection in the seat.

According to one Government source, in the minutes after the announcement, Downing Street was gripped 'by blind panic'. 

Another source told me that the fact that the seat had been supplied by Simons, a former Starmer strategist, 'was a kick in the guts for them. If they'd been able to get their hands on Josh at that moment, they'd have torn him limb from limb'.

Once the initial anger had passed, thoughts turned to what could be done to save the situation.

And in particular, whether Burnham's selection could be blocked by Labour's NEC. 'The immediate view was: 'We blocked him once, we'll just do it again', a minister told me.

It was at that point that Starmer was contacted by Lucy Powell, the party's deputy leader. Powell is a close ally of Burnham, and as such is regarded with deep suspicion by Starmer's team. She was prepared to deliver some hard, but necessary, home truths.

Powell had spent the previous week canvassing opinion among NEC members, she explained. She had also been liaising with the trade unions, who on Wednesday had issued a statement calling on the Prime Minister to set in train an orderly process for his departure. If Burnham was blocked, she advised, it would be instantly terminal for his premiership.

Since Monday night, when Home Secretary Shabana Mahmood had instructed Starmer to set out a timetable for leaving office, then reportedly released the conversation to the Press - Starmer had been, in the words of one ally, 'determined to dig in. His view was f*** you all'. But on this occasion, he seemed to take on board the advice.

Shortly after his conversation with Powell, Downing Street began to brief the media that the Prime Minister believed Burnham should be free to run in Makerfield. Starmer's supporters on the NEC would be told not to stand in his way, journalists were told.

But within No 10, a different plan was forming. According to a Cabinet minister, Starmer's former chief of staff Morgan McSweeney, who had been coordinating Starmer's defence via conference call, furiously objected to the idea of letting Burnham run. 'He was adamantly opposed,' the minister told me. 'He said that Burnham had to be stopped at all costs.'

At that point, a new operation swung into action to try to sabotage the King of the North's triumphal return to Westminster. 'They were ringing up individual NEC members trying to twist their arms. They were pressuring their people to try to get them to block him.

'But they were saying, 'You can't make it look like we put you up to it. You have to tell people you've come to your decision independently.' '

A second minister confirmed this. 'Yes, they've been trying to influence the NEC again,' they said. 'It's utterly mad. They don't seem to realise that if Andy gets blocked again, it's over for Keir. The party will come for him.'

A second attempt to undermine Burnham was made by trying to delay the tabling of the writ for the by-election. According to a Labour source: 'They think that if they can push it back long enough, then they may be able to buy Keir some more time. They might be able to take some of the heat out of the situation, or maybe even give Reform some more time to mobilise in the seat.'

The responsibility for moving a by-election writ rests with the Government's chief whip, Jonathan Reynolds. And according to another Cabinet minister, he was forced to resist pressure from Starmer allies to 'kick the by-election into touch'.

As one of his Cabinet colleagues revealed to me: 'They were telling him to do whatever he could to delay tabling. But Johnny told them where to go. He held firm.

'He said that the writ would be put down on the normal timetable, and in the normal way.'

It was announced yesterday that the by-election will be held on June 18.

Among ministers, there is growing anger at what they see as Starmer's duplicity and McSweeney's perfidy. There is an understanding that the Prime Minister is under intense personal pressure, and a commonly held belief that Starmer's growing realisation that his time in office is drawing to a close is impacting his judgment.

But they don't understand how Sir Keir cannot recognise the damage that will be done to his reputation and legacy if the covert attempts to undermine Burnham are allowed to continue. 'Keir likes to think of himself as a straightforward, honest guy,' one of his colleagues observed. 'But he isn't really. He can be utterly deceitful when he thinks it is in his political interests.'

Another minister pointed to the return of McSweeney. 'What is he doing letting him back in?' he observed. 'It's crazy. McSweeney is the person who got him into this mess in the first place.' (McSweeney is understood to have been the force behind disgraced Peter Mandelson's appointment as US ambassador.)

Among some of Starmer's allies, there's a growing realisation that the game is now up. 'Andy getting a seat was the final straw,' he told me. 'We didn't see that coming.

We thought he was bluffing. He'll get selected, he'll win the by-election, and that will be it.'

But a minister who counts themselves as a proud 'Bitter Ender' - as Starmer's tiny band of remaining supporters are now known within the PLP - told me: 'Look, if Keir just decided to slink off, it would be completely pathetic. 

It would be weak to walk away when the country is facing all these problems. Is that what we call public service now?'

Maybe it isn't. But on countless occasions in the past, the Prime Minister has been keen to point out that one of the key components in public service is integrity. 

And by publicly committing to allow Burnham to stand, then simultaneously giving licence to his allies privately to chop him off at the knees, Keir Starmer is acting both dishonourably and recklessly.

The news that Burnham had secured a seat sent a surge of optimism coursing through a Labour Party that had been brought close to breaking point by the infighting and dysfunctionality of the past week. As one MP said to me: 'Thank God! Now we can end this.'