Harry Dunn’s parents’ High Court battle against the Foreign Office, which they claim unlawfully obstructed the police investigation into their son’s death, is due to begin on Wednesday.

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 asserted 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 this year – 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.

At a two-day remote High Court hearing, their lawyers will argue the FCDO was wrong to conclude that Sacoolas had diplomatic immunity on the basis of what they have described as a “secret agreement” with the US embassy made in 1995.

More than two decades ago, the US asked to expand the number of staff at RAF Croughton who would effectively be given diplomatic immunity.

The UK agreed but asked the Americans to waive the immunity of all “administrative and technical staff” at the base in relation to “acts performed outside the course of their duties”, which the FCDO accepted.

However, the FCDO argues the waiver of immunity only applied to staff at RAF Croughton at the time of Mr Dunn’s death and not their family members – meaning Sacoolas did have diplomatic immunity at the time of the crash.

At a preliminary hearing in June, the High Court heard that a Foreign Office official raised concerns over the potential for a car crash involving a diplomatic immunity claim 25 years before Mr Dunn’s death.

A briefing note sent to Sir Tony Baldry, then a junior minister, in May 1995 said that “an accident involving the claim of immunity could make the local if not national headlines”.

Ms Charles told the PA news agency at the time that the note which “prophesied” her son’s death had left the family “beyond angry”.

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”.

Ms Charles told PA that the move was a “huge step forward”, but added: “We’ve still got a long way to go – we’ve still got to get Anne Sacoolas back.”

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”.

Speaking before the hearing on Wednesday, Ms Charles told PA the family was “confident” but also “naturally apprehensive”.

She added: “We’re still trying to get through every day. We’re still having to force ourselves to get out of bed every morning. We’re still trying to put one foot in front of another.

“One day, one day we will get that time to sit and reflect.”

The hearing before Lord Justice Flaux and Mr Justice Saini, which is being conducted remotely, is due to start at 10.30am and is expected to last two days.