Ministers believe Keir Starmer has made his last mistake. ‘I think that next week he’ll have to hand in his resignation,’ one told me this morning. ‘He completely f***d up at Prime Minister’s Questions. He’s lied to the House. Next week everyone will see he’s lied to the House. And that will be it.’

The fatal blunder they were referring to relates to a statement Sir Keir gave to Conservative leader Kemi Badenoch halfway through yesterday’s sparring match. To widespread incredulity across the Commons, he read out a series of quotes from sacked Foreign Office permanent secretary Sir Olly Robbins, which attempted to give the impression he had cleared No10 and the Cabinet Office of applying untoward pressure in relation to Peter Mandelson’s vetting.

The quotes themselves were selective, misleading and, in one instance, even falsely ascribed. But then Sir Keir went further. ‘No pressure existed whatsoever in relation to this case,’ he defiantly asserted.

According to a minister I spoke to yesterday, that statement had created ‘deep unease’ among No10 officials. But by this morning, that unease had morphed into outright ‘panic’. The reason for this, I’m told, is that the Prime Minister had clumsily deviated from a carefully crafted response prepared for him by his team on how to respond to Sir Olly’s allegation on Monday that ‘throughout January, honestly, my office and the Foreign Secretary’s office were under constant pressure’.

I’m told that, in advance of Sir Olly’s appearance at the foreign affairs committee, Sir Keir’s team gathered to produce a rebuttal to briefings circulated by the civil servant’s allies that he and other officials were press-ganged into fast-tracking Mandelson’s appointment. They decided it would be impossible to refute the charge – not least because they were only too aware such pressure had indeed been applied – but decided on another approach. If pressed, the Prime Minister would simply assert that officials should have been robust enough to resist any arm-twisting.

Keir Starmer has put his team in a state of panic after deviating from his lines during PMQs

He may have misled the Commons by claiming no pressure was put on the Foreign Office

This was precisely the line Sir Keir road-tested in his initial statement to the Commons on Monday.

That day, Lib Dem MP Claire Young put this to the Prime Minister: ‘Ditching a tried and tested ambassador for a high-risk one seems odd behaviour for a Prime Minister who claims to be so fond of proper process. Whose idea was it, and who was applying the pressure?’

Sir Keir responded: ‘I reject the idea that any pressure is a good reason not to disclose to the Prime Minister that UKSV [UK security vetting] recommended against clearance for a very senior, sensitive appointment. I simply do not accept that that is an adequate reason, whatever the pressure.’

But then on Wednesday, for reasons even his closest advisers cannot fathom, he opted to abandon that script – and extemporise. With potentially catastrophic consequences.

Much of Westminster’s focus has been on the news that Sir Keir’s former chief of staff, Morgan McSweeney, is to appear next week before Dame Emily Thornberry’s select committee. But within Downing Street the focus – and fear – is centred on an expected appearance by Sir Philip Barton, Sir Olly’s predecessor.

It is common knowledge within No10, the Cabinet Office and the Foreign Office that Sir Philip was placed under enormous pressure to fast-track Mandelson’s appointment. According to one report, he was ordered by McSweeney to ‘just f*****g make the appointment’.

It’s not clear whether Sir Philip will confirm that specific quote, and McSweeney’s allies insist he would not use such industrial language towards a senior civil servant.

But Downing Street officials do expect Sir Philip to confirm severe pressure was applied. At which point the Prime Minister will be seen to have directly misled Parliament.

‘Barton will deliver the coup-de-grace’, a minister told me. ‘When he directly contradicts what Keir said in the House, there’s no way out.’

The view that Sir Keir’s exit will come as early as next week is not widely shared. The consensus amongst MPs and officials I’ve spoken to is that when Sir Philip’s damning evidence is set alongside the staggering PMQs blunder, it will prove the catalyst for the establishment of a formal inquiry by Parliament’s standards committee. And that could, in turn, be the trigger for Sir Keir’s resignation.

As one minister explained: ‘It was the standards committee that brought Boris down in disgrace. And Keir won’t want that. The optics and parallels will be too damaging. He isn’t going to allow it. He’ll just walk.’

I was talking to another MP yesterday who had watched Sir Keir’s disastrous PMQs performance with mounting horror. I asked them what they thought had provoked the Prime Minister into making such a bizarre blunder. Especially given it came in the middle of a monologue which involved him castigating Badenoch for accusing him of misleading the House and country.

‘I think, subconsciously, there’s part of him that wants it to be over now,’ they said. ‘I think he wants the decision to be taken out of his hands.’

Next week, the Prime Minister may well get his wish.