Sir Olly Robbins was fired in April after it emerged that Lord Mandelson, Starmer’s appointee as US ambassador, had been granted security clearance despite concerns raised by vetting officials. The prime minister later said he was “furious” he had not been informed of the vetting process at the time.

In a statement announcing the legal action, the FDA said the decision to sack Robbins was based on a “grievous misunderstanding” of government vetting procedures. “Rather than being under a duty to tell ministers about the process leading up to the vetting decision, Olly was under an obligation not to,” the union said. “Not only was there no fair procedure involved in his dismissal, there was no process at all.”

The union said it would also seek to argue that Starmer lacked the legal authority to dismiss Robbins, who was permanent under-secretary and head of the diplomatic service.

Starmer has previously said he sacked Robbins after not accepting his explanation for why clearance was granted. “I did ask him, and I did not accept his explanation. That is why I sacked him,” the prime minister told MPs in April.

Robbins, who was appointed to the Foreign Office job in early 2025, has defended his role. He told MPs that he had not seen the documentation from UK Security Vetting (UKSV) containing their assessment of Mandelson. Instead, he said, he received an oral briefing in which the vetting agency described Mandelson as a “borderline case” and suggested the Foreign Office could grant clearance with “appropriate risk management.”

The vetting controversy dates to December 2024, when Mandelson was announced as the UK’s ambassador to the US before in-depth vetting had been completed. He formally took up the role in February 2025 after being granted clearance. Seven months later, he was sacked after further information emerged about his previous friendship with the late convicted sex offender Jeffrey Epstein.

In a statement, Robbins said he was bringing legal action “reluctantly” and that it would have been unnecessary if the prime minister had “simply apologised for his mistake and made amends for the distress and cost it has caused me and my family.”

A government spokesperson said: “We do not comment on legal proceedings.”

Robbins previously served as the UK’s lead official during Brexit negotiations with the European Union.