Harry Dunn’s parents will find out whether they have won their High Court battle against the Foreign Office on Tuesday morning.
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”.
Lord Justice Flaux and Mr Justice Saini will hand down their judgment on Mrs Charles and Mr Dunn’s legal challenge on Tuesday morning.
At a hearing earlier this month, their lawyers 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”.
Sam Wordsworth QC told the court that Sacoolas had “no duties at all” at the base and therefore “never had any relevant immunity for the US to waive”.
Mrs Charles and Mr Dunn’s case centres on a 1995 agreement between the UK and the US, granting immunity to administrative and technical staff at RAF Croughton, which the US waived in relation to “acts performed outside the course of their duties”.
But the FCDO says that waiver only applied to staff at RAF Croughton and not their family members, meaning Sacoolas did have immunity at the time of the crash.
Sir James Eadie QC, for the FCDO, said: “As a matter of international and domestic law, Mrs Sacoolas automatically had diplomatic immunity as the spouse of a member of the administrative and technical staff of the US mission.”
He added that 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”.
Mrs 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 High Court ruling is due to be delivered remotely at 10am on Tuesday morning.