LONDON — The United States has formally turned down Britain’s extradition request for an American woman who was involved in a car accident that killed a teenager last year, a decision that the British government called “a denial of justice.”

The police have said that the woman, Anne Sacoolas, was driving a car on the wrong side of the road in August when it collided with a motorcycle ridden by Harry Dunn, 19. She fled Britain shortly afterward.

At the time of the accident, which occurred in Brackley, a town about 60 miles northwest of London, Ms. Sacoolas’s husband was working for the United States government at a British military base, and American officials assert that she had diplomatic immunity, shielding her from prosecution. But in December, British prosecutors charged her with causing death by dangerous driving.

The State Department said in a statement on Thursday that it had denied extradition, which it said “would render the invocation of diplomatic immunity a practical nullity and would set an extraordinarily troubling precedent.”