Britain agreed to let Anne Sacoolas, the driver charged with killing 19-year-old motorcyclist Harry Dunn, return to the US on the basis of an “apparently illogical” interpretation of the law on diplomatic immunity, according to the most senior civil servant at the Foreign Office. The description of this interpretation given to the foreign affairs select committee by Sir Simon McDonald, the permanent under-secretary and head of the diplomatic service, will add fuel to the efforts of the Dunn family to mount a judicial review of the Foreign Office’s decision-making in the case. The Foreign Office accepted a US government claim that as the wife of a CIA agent working at the US intelligence base RAF Croughton in Northamptonshire, Sacoolas had diplomatic immunity. She was allowed to return to the US with her family, days after driving her car on the wrong side of the road on 27 August and crashing into Dunn on his motorcycle. The Crown Prosecution Service has subsequently charged her with death by reckless driving, but the US government has rejected the British extradition request. It has now emerged that the UK Foreign Office lawyers discussed her diplomatic status with their US state department counterparts in the immediate aftermath of Dunn’s death – and initially advised British ministers on 30 August that there was legal ambiguity about her standing. But then, according to a report in the Mail on Sunday, the Foreign Office told the Northamptonshire police on 2 September that Sacoolas had full immunity and no prosecution was possible. So within three full working days of the crash, the Foreign Office had sided with a US interpretation of what McDonald described to the committee as “a recondite bit of law”. Sacoolas left the UK on 16 September. In the fullest high-level explanation of the events subsequent to Dunn’s death, McDonald told the select committee on Tuesday: “In the case of Harry Dunn, the controversy was over an agreement made at the end of the last century over continuing immunities for US diplomats posted at the Croughton annex. “In that agreement the American authorities gave a pre-waiver for accredited diplomats so that was the formal position, but that agreement was silent on the rights of their dependents, and that has been the origin of a lot of the dispute. But our legal advice is that when an agreement is silent on something, then what pertained before still applies – i.e. immunity.” McDonald told committee members this was a “recondite bit of law”, and the interpretation of the law was “a bit illogical”. The illogicality arose since the US and UK lawyers had mutually decided that, even though the immunity of the diplomat working at the base had clearly been waived by the US in the agreement, that of the dependent had not. Chris Bryant, a Labour member of the foreign affairs select committee, said: “It seems surprising the Foreign Office would accept an anomaly such as a dependent of a diplomat enjoying greater immunity than the diplomat himself. “You would have expected at the very least that the Foreign Office would not side with this dubious American interpretation of the law, but instead leap to the defence of a British citizen.” A subsequent internal Foreign Office review has concluded the legal status of dependents at the Croughton base was anomalous, and the UK is now seeking to renegotiate the agreement. McDonald was clear that the only the Foreign Office, and not the police, can determine the issue of someone’s diplomatic immunity: “The police force does not determine immunity. The police ask the Foreign Office about immunity matters.”