The Foreign Office (FCO) has told the family of Harry Dunn that allegations of a misuse or abuse of power by Foreign Secretary Dominic Raab are "entirely without foundation".
The PA news agency understands 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".
It is understood a legal claim issued by Harry’s family on October 25 was met with strong resistance from the FCO – who said any judicial review would be "opposed" and "defended".
The 19-year-old’s parents had requested the FCO "withdraw the advice" provided to Northamptonshire Police surrounding the granting 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 of 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 August 27.
Mrs Sacoolas, the motorist allegedly responsible for the crash, claimed diplomatic immunity and was allowed to return to the US.
The claim against the FCO issued on behalf of Mr Dunn’s parents - Charlotte Charles and Tim Dunn - alleged the granting of diplomatic immunity to Mrs Sacoolas was "wrong in law".
In the FCO’s response, seen by PA, the family were told the allegation was not accepted, and accusations that Foreign Secretary Dominic Raab had "committed misfeasance" were also entirely without foundation.
Family spokesman, Radd Seiger, said the response from the FCO had been received and they were in the process of digesting 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 October 25, 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."
The response continued: "As to the alternative ground of challenge based on Articles 2 and 6 ECHR, it is not accepted that you have identified any basis under the ECHR to criticise the SSFCA’s actions.
"The SSFCA therefore declines to take the action you have invited him to take on behalf of your clients, and would oppose any application for permission to bring a claim for judicial review.
"We therefore invite you not to pursue the proposed claim. In the event that the claim is issued, it will be defended, the application for permission will be opposed, and the Secretary of State will seek his costs for doing so."
In a statement on Twitter, Mr Seiger added: "The FCO relies on two private agreements between the USA and UK dated in 1995 and 2001 to assert that Anne Sacoolas did have diplomatic immunity, in response to Harry Dunn’s family’s claim against them for their unlawful advice to @nothantspolice.
"Having taken legal advice the family’s position is clear that these arrangements have no basis in law and as Dominic Raab himself said in his statement in the House of Commons, have never been tested by the Courts.
"As if it were not enough for the family to have to endure the loss of Harry, the British Government now appear intent on putting them through a needless and protracted legal battle culminating in Court. So be it. They will not rest until justice is done. But shame on the government."