Prime Minister Boris Johnson has said he does not believe diplomatic immunity should be used in the case of a woman allegedly involved in a fatal crash.
A US diplomat’s wife Anne Sacoolas is a suspect in inquiries into the death of motorcyclist Harry Dunn.
He was killed in Northamptonshire on 27 August.
Ms Sacoolas left the UK despite initially telling police she had no such plans.
Under the 1961 Vienna Convention, diplomats and their family members are immune from prosecution in their host country, as long as they are not nationals of that country. However, their immunity can be waived by the state that has sent them.
The US State Department said diplomatic immunity was “rarely waived”.
Speaking during a visit to a hospital in Watford, the Prime Minister said: “I think everybody’s sympathies are very much with the family of Harry Dunn and our condolences to them for their tragic loss.
“I must answer you directly, I do not think that it can be right to use the process of diplomatic immunity for this type of purpose.
“And 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.
“That’s a point that we’ve raised or are raising today with the American ambassador here in the UK and I hope it will be resolved very shortly.
“And to anticipate a question you might want to raise, if we can’t resolve it then of course I will be raising it myself personally with the White House.”
Both Northamptonshire’s chief constable and police and crime commissioner have already urged the Americans to waive Ms Sacoolas’s diplomatic immunity.
Chief constable Nick Adderley said that “based on CCTV evidence”, officers knew that “a vehicle alighted from the RAF base at Croughton” and was “on the wrong side of the road”.
UK Foreign Secretary Dominic Raab has also urged the US Embassy to reconsider.
Mr Dunn’s mother, Charlotte Charles, said leaving the country was “such a dishonourable thing to do” and urged Ms Sacoolas to come back.
Ms Charles told the Victoria Derbyshire programme: “[It was] unintentional. She didn’t purposely drive on the other side of the road… 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.”
The US State Department said it was in “close consultation” with British officials and has offered its “deepest sympathies” to the family of Mr Dunn.