The prime minister has said he will raise the case of Harry Dunn with the White House if a resolution cannot be found any other way.
The 19-year-old was allegedly killed in a crash involving a US diplomat’s wife – she has since left the UK despite telling police she had no such plans.
Asked about calls for Anne Sacoolas to return to face further questioning, Boris Johnson said: “I do not think that it can be right to use the process of diplomatic immunity for this type of purpose.”