LONDON — The diplomatic immunity of Anne Sacoolas, the American woman who is a suspect in a road crash that killed Harry Dunn, a British teenager, is “no longer relevant” in the case, Britain’s foreign secretary has said.

Ms. Sacoolas, the 42-year-old wife of a diplomat, fled Britain using diplomatic immunity after the police in Northamptonshire, England, suspected her of being the driver of a car that they say had been traveling on the wrong side of the road when it collided with a motorcycle ridden by Mr. Dunn, 19, in August.

Britain and the United States have been involved in a diplomatic tug-of-war over the case, but Dominic Raab, the British foreign secretary, said in a letter to the Dunn family on Saturday that Ms. Sacoolas’s immunity no longer applied because she had gone back to the United States.

“The U.K. Government’s position is that immunity, and therefore any question of waiver, is no longer relevant in Mrs. Sacoolas’s case, because she has returned home,” Mr. Raab said in the letter that was shared with The New York Times on Sunday.