The Foreign Office has been accused of "threatening and intimidating" Harry Dunn's family after claiming their legal challenge is "entirely without foundation".
The family of Mr Dunn, a 19-year-old who died in a car collision outside RAF Croughton in August, are taking legal action against Foreign Secretary Dominic Raab.
They allege he misused or abused his power by granting the wife of a US diplomat, Anne Sacoolas, diplomatic immunity.
In an exclusive interview with Sky News, the family's lawyer Mark Stephens said the Foreign Office was "trying to intimidate the family from having their case scrutinised by the courts".
Mr Stephens accused the British government of "acting like the lickspittles of Americans".
In a letter to the family, the Foreign Office said it would "oppose" and "seek costs" for any judicial review brought.
Anne Sacoolas is believed to have been driving on the wrong side of the road when her car hit and killed Harry Dunn.
More from Harry Dunn
The 42-year-old claimed diplomatic immunity after the fatal crash and later left the UK.
In the letter, the Foreign Office said the family have "not identified any reasonably arguable ground of legal challenge".
The Foreign Office letter said: "We therefore invite (Harry Dunn's family) 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".
The family's lawyer said: "I think the Foreign Office know they are in weak legal position that is why they are piling all the pressure on to try to intimidate the family.
"They are trying threaten and intimidate Harry's parents so that not only do they lose their son but they lose their homes as well.