The prime minister’s decision to send Lord Mandelson to Washington is like a horror film for Sir Keir Starmer, stuck on repeat.

The whole saga has now cost another person their job – Sir Olly Robbins, the most senior civil servant in the Foreign Office.

And there are some, both on the opposition benches and within the Labour Party, who think it might yet cost the prime minister his.

Let me walk you through the extraordinary twists and turns of Thursday afternoon and evening.

Shortly after 15:00 BST, the Guardian published its story saying that Lord Mandelson had failed his security vetting clearance, but the decision had been overruled by the Foreign Office.

I immediately made calls to the Foreign Office, Downing Street, the then Foreign Secretary David Lammy’s team and the Cabinet Office. None of them engaged, for nearly three hours.

Usually, when a story isn’t right or has perhaps leapt to conclusions seen to be unreasonable, my phone will be ringing in seconds. But it didn’t.

The opposition parties quickly concluded there must be something in the Guardian’s claims, and one after another they appeared in front of the cameras to claim the prime minister had misled the House of Commons, and if he had done so knowingly, he would have to resign.

As I headed to our live camera to report on this for the BBC News at Six, a statement from the government appeared on my phone.

Neither the prime minister nor any minister had any idea this was conclusion that had been reached, it said.

Those opposition parties returned to the cameras and microphones.

How on Earth could he have been so lacking in curiosity about the process, they asked.

The prime minister will address Parliament, probably on Monday, to address this – and what he knew and when. I’m told he found out on Tuesday evening, as part of the government’s process of going through documents about Lord Mandelson that Parliament has demanded are published.

Sir Keir is, I understand, absolutely furious. Several figures we have spoken to who worked in No10 at the time insist they had no idea about all this. Friends of Morgan McSweeney, the prime minister’s chief of staff at the time, say he didn’t know either.

And, incidentally, I’m told Lord Mandelson himself wasn’t aware too.

The suggestion in government is the Foreign Office knew, but didn’t tell anyone – or at the very least failed to ensure that its own foreign secretary or the prime minister were informed.

It is this that has led Sir Keir and the Foreign Secretary, Yvette Cooper, to give Sir Olly Robbins the heave ho. We are yet to hear from Sir Olly himself.

Some have pondered if a subtlety here may be that the vetting advice wasn’t definitive in its judgement, but the Foreign Office concluded it amounted to Lord Mandelson failing it and others did not. But that doesn’t explain why that conclusion from the Foreign Office wasn’t passed on.

The BBC team has been in touch with Labour MPs to see what they make of all this.

“I think we’ve now reached the stage where the prime minister was blissfully unaware is a good explanation. That’s where we are,” reflected one.

“Lost for words,” said another.

“Surely the cabinet now see it’s dead,” said a third, a long-standing critic of Downing Street, about the prime minister’s future, implying he should not have much of one in office.

This is, quite frankly, the last thing the prime minister needs. And it won’t be the last of all this.