British prosecutors have requested the extradition of Anne Sacoolas over the crash last August in which 19-year-old Briton Harry Dunn was killed while riding his motorbike.

But the State Department said on Thursday that Sacoolas had diplomatic immunity and that to extradite her would set "an extraordinarily troubling precedent".

British Foreign Secretary Dominic Raab said on Friday that he had spoken to the US ambassador in London, Woody Johnson, to express Britain's disappointment.

"We feel this amounts to a denial of justice, and we believe Anne Sacoolas should return to the UK," Raab added.

"The UK would have acted differently if this had been a UK diplomat serving in the US."

"We are now urgently considering our options."

In a Twitter post, Johnson expressed his "sincere condolences and sympathy to the Dunn family for their loss".

Dunn's family have said Sacoolas was driving on the wrong side of the road at the time of the crash, near an air force base in central England used by the U.S. military.

Sacoolas left Britain shortly after the accident. Her lawyer has said that she will not return voluntarily to potentially face jail for "a terrible but unintentional accident".

However Dunn's mother Charlotte Charles said Sacoolas had to return to Britain.

"Diplomatic immunity or not, like we've said all along it still doesn't cover somebody to run away from laws in our country," she told reporters

"She will come back, I don't care how long it takes."

British Prime Minister Boris Johnson has said Sacoolas was wrong to use diplomatic immunity to leave Britain and has urged US President Donald Trump to reconsider the American position.

Dunn's parents met Trump at the White House in October. Trump hoped to persuade them meet to Sacoolas, who was in the building at the same time, but they declined.

Australian Associated Press