The Foreign Office has been accused of "threatening and intimidating" the family of Harry Dunn after claiming that their court challenge was "totally unfounded".
The family of Mr. Dunn, a 19-year-old man who died as a result of a car accident outside the RAF Croughton in August, has initiated legal proceedings against Foreign Secretary Dominic Raab.
They allege that he abused or abused his power by granting diplomatic immunity to the wife of a US diplomat, Anne Sacoolas.
In an exclusive interview with Sky News, family lawyer Mark Stephens said the Foreign Office "was trying to intimidate the family so that their case would be reviewed by the courts".
Mr Stephens accused the British government of having "acted like the stick of the Americans".
In a letter to the family, the Foreign Office stated that it would "oppose" and "seek costs" for any application for judicial review.
It is believed that Anne Sacoolas was driving on the wrong side of the road when her car hit and killed Harry Dunn.
Aged 42, he claimed diplomatic immunity after the crash, before leaving the UK.
In that letter, the Foreign Office stated that the family "has not identified any grounds for a reasonably arguable court challenge".
The Foreign Office letter stated: "We therefore invite (the family of Harry Dunn) not to respond to the request." In the event that the request is made, it will be defended, the request for authorization will be challenged and the secretary of state look for his expenses to do it ".
The family lawyer said: "I think the Foreign Office knows that he is in a precarious legal position, which is why he is putting all the pressure on him to try to intimidate his family.
"They are trying to threaten and intimidate Harry's parents so that they lose not only their son but also their home.
"With regard to the legal argument, it is clear that the Vienna Convention on Diplomatic Immunity does not apply.
"The British government made a simple promise to the Americans: if their diplomats committed job-related crimes, they would enjoy immunity, which does not apply to his wife driving in a car.
"If you kill someone or if your family kills someone, diplomatic immunity does not apply."
In a statement, a Foreign Office spokesman said: "We have deep sympathy for Harry's family.
"We have and will continue to do everything in our power to ensure justice.
"As stated by the Minister of Foreign Affairs in Parliament, the subject enjoyed diplomatic immunity when he was in the country under the Vienna Convention on Diplomatic Relations. "