Olly Robbins has just destroyed Keir Starmer’s premiership. He didn’t arrive at the foreign affairs select committee brandishing a smoking gun. He carried a smoking bazooka.

Yesterday, the Prime Minister spent more than two hours telling the House of Commons and the country he was ‘furious’ to learn that Peter Mandelson had failed vetting. And that if he had known he had failed vetting, he would never have appointed him.

Robbins exposed the truth. No 10 actually had a ‘dismissive’ attitude towards the vetting process. In fact, they had even attempted to get Mandelson sent to Washington without any vetting process at all. Robbins and other Foreign Office officials were placed under daily pressure from Downing Street to force through his appointment, he revealed.

In September last year, Keir Starmer had also told the House that ‘due process’ had been followed at all times over Mandelson. Robbins proved beyond doubt that the statement was a 100 per cent, copper-bottomed lie. In truth, Downing Street had done everything it could to override the process and force Mandelson into the most sensitive post in the British diplomatic service without any proper due diligence, and against the concerns of the intelligence services, who believed Mandelson represented a major national security risk.

Yesterday, it emerged that the former Cabinet Secretary Simon Case had advised Keir Starmer that he should wait for Mandelson to obtain the proper security clearances before appointing him. Starmer had ignored that advice, but claimed there was no issue with his decision. Again, Robbins destroyed that argument. He specifically said it was important to secure the clearances prior to the appointment of the Washington ambassador because of the importance the United States placed on intelligence sharing. And that he, too, would have strongly advised the Prime Minister to ensure vetting should be completed before Mandelson’s appointment was confirmed.

Before he was elected, Keir Starmer vowed to end the conveyor belt of cronyism. Yet Robbins revealed that he had been called by No 10 with a request to see if Matthew Doyle, Starmer’s director of communications, could be lined up with a plum overseas role. Robbins was told by No 10 to hide this request from the Foreign Secretary.

Yesterday, Keir Starmer pointedly refused to admit to the House of Commons that he had even inadvertently misled MPs. Robbins’s evidence made clear that, to be fair to Starmer, that was correct. He hadn’t inadvertently misled them. As Robbins made crystal clear, he’d deliberately and repeatedly lied through his teeth to them.

Thanks to the former permanent secretary’s un-Sir-Humphrey-like candour, we finally have an accurate picture of the process that surrounded Mandelson’s appointment. Due process was not followed. Nothing like due process was followed.

Instead, Keir Starmer decided to appoint his colleague and friend as his most senior diplomat. He knew the issues surrounding Mandelson. His relationship with Epstein. His business links with Russia, and allies of Putin. His lobbying on behalf of the Chinese regime.

We know he knew, because he was handed a due diligence document that detailed them all. We know he knew because he was warned about them by his officials. He was warned about them by his National Security Adviser. He was warned about them by his Foreign Secretary. This morning, his Energy Secretary Ed Miliband went on the morning broadcast round to confirm he also warned about them.

And Starmer ignored those warnings. Not only did he ignore those warnings, but he and his advisers took the decision to force Mandelson into the post, come what may.

The due diligence process was corrupted. Direct pressure was placed on Foreign Office officials not just to expedite Mandelson’s appointment, but to ensure he was put in place regardless of the security and blackmail risks. Indirect pressure was placed on those officers tasked with deciding whether to grant him access to the most highly classified secrets of the United Kingdom and her allies.

And that appointment promptly blew up in the Prime Minister’s face. At which point the cover-up began.

Parliament was deliberately misled over the process surrounding Mandelson’s appointment. When it finally realised and rebelled, an attempt was made to ‘disappear’ the evidence MPs had demanded. Phones mysteriously vanished. Documents were deleted. Messages erased.

And yet despite the increasingly frantic efforts of the Prime Minister and his allies to bury the truth, it has stubbornly refused to remain interred. Which leaves Keir Starmer facing a choice.

He can continue to put his MPs, party and the country through this grotesque farce. He can try to cling to the surreal fiction that he did nothing wrong. Repeatedly demand that everyone but himself pay the price for the Mandelson scandal. Mouth some fresh ritualistic self-serving words of apology to Epstein’s victims, then try to shrug the whole affair off.

Or he can finally realise the game is up. Robbins has finished him. The faux anger. The mock contrition. The lawyerly evasion. It has run its course.

‘I know many MPs will find these facts incredible,’ the Prime Minister told the Commons yesterday, to audible ridicule. But he was right. His statements about Peter Mandelson lack any credibility. And when a Prime Minister no longer has credibility, there is only one course of action left.