Finally it's over. After another weekend which began with Keir Starmer barricaded in Chequers with his loyal advisers, and explaining to anyone who will listen how he was experiencing another tortuous, dark night of the soul, reality has finally bitten. And bitten deep.

Despite all the bombastic briefings, the fight has finally been extinguished from our embattled Prime Minister.

There is as yet no formal announcement. But numerous government sources have confirmed that tomorrow Sir Keir will at last announce he is stepping down. The reason for his decision is not yet clear. However, according to one insider I spoke to, 'something happened today. He was going to take more time to think it over but something convinced him there was no point fighting on.'

On Friday I was talking to a former Starmer aide, asking whether they thought their former boss would finally bow to political reality. The former aide told me: 'To be honest it depends on what Morgan [McSweeney, his former chief of staff] and his wife Victoria say to him. They're basically the only two people he listens to. They're his only properly close friends.'

It was certainly the case that by the end he had few friends left among his parliamentary colleagues. In the hours after Andy Burnham's triumph in Makerfield, Sir Keir reached out to those members of his Cabinet he regarded as most loyal. Their response shocked him.

'He was expecting them to say they thought he should fight on, and was preparing to set out his strategy,' a minister revealed. 'When they told him they thought it was time to set out a timetable, he was stunned.' Among those who are believed to have told the Prime

Minister some unpalatable home truths are previous supporters Heidi Alexander, David Lammy and Bridget Phillipson.

Though if they were hoping their entreaties to place party and country ahead of Starmer's own vanity and ambition would move him, they were to be sorely disappointed. 'He's in a volcanic rage,' one ally revealed to me. 'He thinks he's been betrayed. Not so much by the Cabinet, but by Andy, and in particular Wes Streeting. He's spitting blood over them.'

The picture Downing Street had sought to paint is of Starmer – determinedly and stoically – soldiering on in the face of mounting adversity. But the truth is that, as the end of his premiership drew to its close, he had retreated into petulant isolation.

It was not just that the Prime Minister was seemingly refusing to listen to reason. He was no longer prepared to even acknowledge the counsel of anyone who didn't tell him precisely what he wanted to hear – namely the fiction the nation simply could not cope without him.

With the result that the last two people left in the No 10 bunker with him at the end were his previously discarded chief of staff McSweeney and his own wife.

McSweeney, who spent the past 48 hours circulating a briefing document that purported to show how Starmer could best Andy Burnham in a future leadership contest, had long been hyped by many of the Prime Minister's supporters as some great political Machiavelli.

But, based on his recent interventions, the reality is he's either a political imbecile or an ideological lunatic – or quite possibly both.

There was no conceivable path to survival for the Prime Minister. Nor any way of derailing Andy Burnham. Had Starmer tried to stand in the way of the on-rushing Burnham juggernaut he would have become Whitehall roadkill.

His Cabinet would have united against him. His MPs would have united against him. The unions would have united against him.

Every Labour affiliate from the Fabians to the Woodcraft Folk would have united against him.

At which point the Labour Party membership would then have risen up as one, and slung him unceremoniously out of office.

And the fact that Keir Starmer's former chief of staff genuinely couldn't see that till the bitter end goes a long way to explaining why his boss has been consigned to political oblivion.

Then there was the pivotal Victoria. Over the past 24 hours, the PM's allies were fanning out to brief that she would be the decisive voice in determining whether he stepped down, or opted to fight on. 'Vic is of the view, "You need to keep on going,"' one pro-Starmer aide briefed yesterday. Another claimed she is 'his rock'.

'You hear second-hand she's really pushing for him to stay.'

Maybe. But with the greatest respect to Lady Starmer, that was never her decision to make.

When it was reported that Boris Johnson's wife Carrie was interfering in his government, she was branded Carrie Antoinette and Princess Nut Nut.

But by the end all Sir Keir had to fall back on was a forlorn hope that the nation would sit back and admire the way Lady Starmer was dictating who its Prime Minister was.

Fortunately, in the end wiser heads have prevailed. But not quickly enough to save Sir Keir from the humiliation of being effectively driven out of office.

Over the weekend I spoke to another minister who claimed that, deep down, Keir Starmer did realise his premiership was over, and was simply looking for a way to step down with honour.

But that opportunity was squandered. He could have stepped down when the Peter Mandelson scandal broke.

He could have done so when he was soundly rejected by the voters at May's local elections.

He could have fallen on his sword when his Defence Secretary resigned, having castigated him for leaving Britain at the mercy of our enemies.

Yet he chose not to. Keir Starmer clung on desperately to power in the hope that something would come up to save his political skin.

With the result the man who entered office pledging to restore faith in public office ended up exhibiting the same selfish egotism and his predecessors.

I have been one of the Prime Ministers sternest critics. And I have long argued that people need to distinguish between the politician and the man.

The Keir Starmer I have encountered has always been a good, decent and compassionate person.

But this morning that calculus needs to be reversed. However decent Starmer may be, over the last few months he has put Britain through unnecessary purgatory.

He had lost the faith of the British people long before he was prepared to acknowledge it. And he has clung to power well past the point that dignity and duty demanded.

Keir Starmer wanted to leave office with honour. But the harsh truth is he waited too late.