Britishforeign minister Dominic Raab is set to call US Secretary of State Mike Pompeo over the case of an American diplomat's wife who used diplomatic immunity to leave the UK after being involved in a fatal car crash.
Prime Minister Boris Johnson said he hoped Anne Sacoolas, wife of Jonathan Sacoolas, would return to the UK voluntarily and "engage properly with the processes of law as they are carried out in this country".
The concerns the case of Harry Dunn, 19, who died in August after a road collision near RAF Croughton, an air force base in Northamptonshire in central England that is used by the US armed forces.
Johnson said the UK government had already raised the issue with the US ambassador in London but if they could not resolve it he would "raise it personally with the White House".
A spokesman for the U.S. Embassy in London said: "Any questions regarding a waiver of the immunity with regard to our diplomats and their family members overseas in a case like this receive intense attention at senior levels and are considered carefully given the global impact such decisions carry.
"Immunity is rarely waived. The U.S. Embassy has been and will continue to be in close contact with appropriate British officials."
Northamptonshire police's chief constable, Nick Adderley, told BBC television that investigators knew that a vehicle which left the RAF base was on the wrong side of the road at the time of the accident.
Dunn'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.