The family of teenage motorcyclist Harry Dunn have launched a legal action against the Foreign Office which they said could cost them “upwards of £50,000”.

The 19-year-old’s parents submitted a judicial review against the foreign secretary, Dominic Raab, on Wednesday following a long-running dispute over the extension of diplomatic immunity to intelligence staff and their families at RAF Croughton.

Harry was killed when his motorbike was involved in a head-on crash with a car outside the base in Northamptonshire on 27 August.

Anne Sacoolas, the motorist suspected of driving the car, claimed diplomatic immunity and was allowed to return to the US.

The FCO has said it would “oppose and seek costs” for any judicial review and that claims of any misuse or abuse of power by Raab were “entirely without foundation”.

Harry’s family has claimed the FCO had “no legal power to make such an agreement” over the immunity.

They have claimed secret treaties between the UK and US that give intelligence staff at the base legal protection have now been disclosed to them and that the documents make no reference about immunity for family members.

The family’s spokesman, Radd Seiger, said the case was “likely to be appealed” by the FCO and if the case goes to the supreme court, it will cost them upwards of £50,000.

The family’s claim for a judicial review came two days after the teenager’s father, Tim Dunn, travelled to the Esher and Walton constituency to confront Raab at a hustings event.

The claim against the FCO issued on behalf of Dunn’s parents alleged the granting of diplomatic immunity to Sacoolas was “wrong in law”.

They requested the FCO withdraw advice provided to Northamptonshire police surrounding the granting of diplomatic immunity to the suspect.

The family have said they are concerned Raab was “pressured by the United States to interpret the law in a way which allowed her to escape justice”.

Seiger said: “The parents have done everything physically within their power to avoid having to sue the FCO.

A spokesman for the FCO said: “We have deep sympathy for Harry’s family. We have done and will continue to do everything we properly can to ensure that justice is done.

“As the Foreign Secretary set out in Parliament, the individual involved had diplomatic immunity whilst in the country under the Vienna Convention on Diplomatic Relations.”