Boris Johnson says the American woman implicated in the death of motorcyclist Harry Dunn should face justice in the UK.
The Prime Minister believes the refusal of the US to extradite Anne Sacoolas is a “denial of justice”.
Mr Johnson reiterated the need for action in a phone call with President Donald Trump on Friday.
A Downing Street spokeswoman said:
“The Prime Minister raised the tragic case of Harry Dunn and the need to secure justice for Harry’s family.
“We believe that this is a denial of justice and the individual concerned should return to the UK.”
Mr Dunn, 19, died after his motorbike and a car collided outside a US Military base in Northamptonshire.
The car driver was Mrs Sacoolas, the wife of a US diplomat.
She promptly returned to America claiming diplomatic immunity.
Britain has asked for extradition but American has refused.
The US Secretary of State Mike Pompeo is in London this week meeting Mr Johnson and the Foreign Secretary Dominic Raab.
It’s believed the case of Mrs Sacoolas will be raised once more.
However, a spokesman for the US State department has said if the extradition request was granted, it would “render the invocation of diplomatic immunity a practical nullity”.
He added it would “set an extraordinarily troubling precedent”.