Make no mistake: when Sir Olly Robbins appears before a parliamentary committee next week, he could well be carrying with him both the means and the opportunity to detonate what remains of Sir Keir Starmer’s already battered premiership.

Robbins is no stranger to political wreckage. He was chief Brexit negotiator for Theresa May. Their doomed scheme led to her tearful departure from Downing Street in 2019.

At 6ft 3in, he is certainly the tallest – and perhaps the most inconvenient – sacrificial lamb yet in Starmer’s growing flock. And Starmer knows only too well that the notoriously ruthless civil servant could yet unleash all he knows about Peter Mandelson’s appointment.

Even Robbins’ fiercest critics – who have long accused him of sabotaging Brexit – are struggling to swallow Downing Street’s line that he deliberately kept Starmer and other ministers in the dark.

The timeline of his appointment alone raises eyebrows. Robbins, 50, became the Foreign Office’s top mandarin in January 2025 – three weeks after Peter Mandelson had been appointed to the most prestigious post in the British diplomatic service, but before in-depth vetting had been carried out.

We know what happened next: Mandelson was denied clearance by the vetting team, a decision that was overruled by the Foreign Office two days later. Mandelson was removed from the post in September last year over his links to the paedophile Jeffrey Epstein.

When a committee of MPs asked in November whether the Foreign Office held a ‘different view’ from No 10 on Mandelson’s appointment, Robbins was careful in his response, saying it was clear that ‘the Prime Minister wanted to make this appointment himself’.

But when it came to the mechanics of the vetting process, Robbins insisted that he was operating by the letter of the law. The Constitutional Reform and Governance Act 2010 allows final calls on difficult vetting decisions to rest with permanent secretaries.

Sir Olly Robins, who was ousted as Foreign Office boss on Thursday, was chief Brexit negotiator under Theresa May

Robbins – in the full knowledge that Starmer had decided Mandelson was the man for the US job – used that authority to overrule concerns over the appointment. His decision to do so is now at the heart of the political storm.

Was it an independent decision or was he leant on? One theory put forward by Starmer’s diminishing band of supporters is that Robbins was politically aligned with the Prime Minister over the appointment. That both of them believed only Mandelson had the guile – and the heft – to strong-arm Donald Trump into accepting the contentious Chagos Islands deal, which would have seen Britain hand sovereignty to Mauritius at a cost to the taxpayer of £35billion.

By the time the security clearance fiasco leaked on Thursday, Robbins knew the game was up. But that night, he received a raft of telephone calls from colleagues warning him to resist when Starmer inevitably threw him under a bus. Yet Robbins fell on his sword.

So could a seasoned operator like him really have taken such a seismic decision without telling any ministers? Even Labour ministers and MPs I spoke to yesterday find that hard to believe. ‘He is a professional civil servant. I think he was forced into this,’ said an impeccable Whitehall source.

Whatever the case, Olly Robbins is no stranger to finding himself in the firing line. As Brexit negotiator from 2017 to 2019, he was blamed for a string of concessions to Brussels. On Tory benches, he earned the moniker ‘the Brexit Betrayer’.

Robbins’ Europhile instincts run deep. At Oxford – where he studied politics, philosophy and economics – he was secretary of the university’s fledgling Oxford Reform Club, established to oppose the Eurosceptic movement. He has previously said: ‘There is no part of my personal views that will ever play a role in how I serve the Government of the day.’ But few Brexiteers believe that.

Which brings us to his appearance at Tuesday’s foreign affairs committee. He will be alongside Sir Chris Wormald, the former Cabinet Secretary ousted by Starmer in February. Both will face forensic questioning about the security vetting. The diplomat’s answers to those questions could decide Starmer’s fate.