Diplomatic immunity for Harry Dunn’s alleged killer was “anomalous” but was still in force at the time of the fatal crash, the High Court has heard.

Mr Dunn, 19, was killed when his motorbike crashed into a car being driven on the wrong side of the road by American Anne Sacoolas outside RAF Croughton in Northamptonshire on August 27 last year.

Sacoolas, whose husband Jonathan Sacoolas worked as a technical assistant at the base, left the country a few weeks later after the US said she was entitled to diplomatic immunity.

The 43-year-old was ultimately charged with causing death by dangerous driving last December, but an extradition request was rejected by the US State Department in January – a decision it later described as “final”.

Mr Dunn’s parents, Charlotte Charles and Tim Dunn, claim the Foreign, Commonwealth and Development Office (FCDO) wrongly decided Sacoolas had diplomatic immunity and unlawfully obstructed Northamptonshire Police’s investigation into their son’s death by keeping the force “in the dark”.

On the second day of a remote hearing on Thursday, the FCDO’s barrister Sir James Eadie QC said rules on diplomatic immunity and any waivers were “super clear”.

In written submissions, he said Sacoolas “automatically had diplomatic immunity as the spouse of a member of the administrative and technical staff of the US mission”.

In 1995, the UK agreed to an American request to include staff at RAF Croughton on the diplomatic list, but asked the US to waive the immunity of administrative and technical staff in relation to “acts performed outside the course of their duties”.

Sir James said the US “expressly waived the immunity from the UK’s criminal jurisdiction of ‘employees’ or ‘staff members’”, but “at no point is there a waiver of the immunity enjoyed by the families of such individuals”.

He argued that the waiver “did not cover family members” so Sacoolas did have immunity at the time of the crash.

In July, Foreign Secretary Dominic Raab announced that “the US waiver of immunity from criminal jurisdiction is now expressly extended to the family members of US staff” at the base, “ending the anomaly in the previous arrangements”.

Sir James told the court there was “nothing inconsistent between the clear effect” of the 1995 agreement and Mr Raab “acknowledging that the effect of the arrangements, correctly analysed, was anomalous”.

In a witness statement previously before the court, Hugo Shorter – the FCDO’s director for the Americas – said officials thought the US was “exploiting the wording” of the agreement shortly after Mr Dunn’s death, but that “there was nothing that the UK could do to prevent (Sacoolas’) return to the US”.

Sir James also said FCDO officials had “objected in strong terms” to Sacoolas leaving the UK, and “repeatedly emphasised” that the department “wanted the Sacoolas family to co-operate with the UK authorities”.

He added: “Since Mrs Sacoolas’s departure, the UK has communicated its disappointment to the US on multiple occasions, and at the highest levels, and repeated the position that Mrs Sacoolas should engage with the UK criminal and judicial process.”

On Wednesday, Geoffrey Robertson QC – representing Ms Charles and Mr Dunn – said the FCDO “took upon itself the authority to resolve the question of immunity and ultimately and unlawfully decided to accept the US embassy’s decision that Anne Sacoolas had immunity”.

He said that decision “obstructed the police by preventing any effective further progress in its investigation into Harry’s death and likely prosecution of Anne Sacoolas”.

Mr Robertson argued the FCDO “tacitly accepted the Sacoolas family’s departure from the UK”, referring to a text message sent to a US embassy official on September 14 2019 – a day before Sacoolas and her family left the UK.

The message read: “I think that now the decision has been taken not to waive (immunity), there’s not much mileage in us asking you to keep the family here.

“It’s obviously not us approving of their departure but I think you should be able to put them on the next flight out.”

But Sir James said the person who sent that message “had the previous day informed the recipient in person of the UK’s strong objections to the US’s intended course of action, but it was clear that there was no realistic prospect of convincing the US to change its approach”.

Ms Charles and Mr Dunn initially also took legal action against Northamptonshire Police but that claim was dropped in July, with the family’s spokesman saying the force had been “absolved of any blame”.

The hearing before Lord Justice Flaux and Mr Justice Saini is due to conclude on Thursday afternoon and it is expected the court will reserve its judgment to a later date.