The Foreign Office has told the family of Harry Dunn that allegations of misuse or abuse of power by the foreign secretary, Dominic Raab, are “entirely without foundation”.

According to reports, the FCO will “oppose and seek costs” for any judicial review brought because the family have “not identified any reasonably arguable ground of legal challenge”.

A legal claim issued by Harry’s family on 25 October was met with strong resistance from the department.

The 19-year-old’s parents had asked the FCO to “withdraw the advice” provided to Northamptonshire police concerning the provision of diplomatic immunity to the US suspect in the case, Anne Sacoolas.

The family’s lawyers also offered an alternative request of “paying substantive damages” for breaching the European convention on human rights.

The teenager was killed when his motorbike was involved in a head-on crash with a car outside RAF Croughton, in Northamptonshire, on 27 August 2019.

Sacoolas, the motorist allegedly responsible for the crash, claimed diplomatic immunity and was allowed to return to the US. She is the wife of a US intelligence officer.

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

In the FCO’s response the family were told the allegation was not accepted, and that accusations that Raab had “committed misfeasance” were also entirely without foundation.

The family’s spokesman, Radd Seiger, said the response from the FCO had been received and they were in the process of considering it with lawyers.

In the letter, the FCO wrote: “It is not accepted that the proposed claim for judicial review articulated on your clients’ behalf in your letter, dated 25 October, identifies any reasonably arguable ground of legal challenge.

“It is not therefore accepted that you have identified any arguable basis on which to suggest that the FCO ought now either ‘to withdraw the advice provided to the police’, or to pay ‘substantive damages for the breach of articles 2 and 6, ECHR’.

“The unparticularised allegation that the secretary of state for foreign and commonwealth affairs (SSFCA) has misused and/or abused his power and/or has committed misfeasance in public office is entirely without foundation.”