Harry Dunn, 19, of Charlton, Banbury, died on August 27 after his motorbike collided with a car near RAF Croughton, an air force base in Northamptonshire in central England that is used by the United States.
The main suspect in the case has been identified by British media as 42-year-old Anne Sacoolas and reports suggest she recently left the UK using diplomatic immunity.
Under the 1961 Vienna Convention, diplomats and their family members cannot face prosecution in the host country, as long as they do not hold the nationality of that country. However, there is also a provision that allows for their immunity to be waived by the state that has sent them.
Sacoolas is said to have left the UK despite telling police she had no such plans.
"The Foreign Secretary spoke with Secretary Pompeo today to discuss a range of issues including Brexit, Hong Kong, Syria, Iran and the case of Harry Dunn where he reiterated his disappointment with the US decision and urged them to reconsider," a Foreign Office spokesman said.
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Earlier, UK Prime Minister Boris Johnson said it was wrong for Sacoolas to have used diplomatic immunity to leave the country, and that he wants her to return to Britain.
"I hope that Anne Sacoolas will come back and will engage properly with the processes of law as they are carried out in this country," Johnson said in a television interview, adding the issue was being raised with the US ambassador in London.
"Immunity is rarely waived. The US Embassy has been and will continue to be in close contact with appropriate British officials."
Northamptonshire police’s chief constable, Nick Adderley, told BBC that investigators knew that a vehicle that left the RAF base was on the wrong side of the road at the time of the accident.
The victim's mother told the broadcaster she wanted the diplomat’s wife to return to face justice.
"If she’d have stayed and faced us as a family we could have found that forgiveness ... but forgiving her for leaving, I’m nowhere near," Charlotte Charles said.
The US Embassy spokesman said: "We express our deepest sympathies and offer condolences to the family of the deceased in this tragic traffic accident."
He declined to comment on the identity of the driver.
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