The wife of a US diplomat on Thursday pleaded guilty over a road crash that killed a teenaged British motorcyclist and has dogged US-UK relations.

Harry Dunn, 19, died in August 2019 when his motorbike collided with a car driving on the wrong side of the road near a US airbase in southern England.

Sentencing for the suspect, Anne Sacoolas, is due next month after she entered her plea via video link from Washington to a London court.

Sacoolas, whose husband was an intelligence official and has herself been reported to have been a CIA operative, left Britain soon afterwards and claimed diplomatic immunity.

Since the accident, Dunn's parents had been leading a high-profile fight to achieve justice for their son while US authorities refused her extradition.

She was originally charged with causing death by dangerous driving but pleaded guilty to a lesser charge of careless driving which can bring a maximum jail term of up to five years.

Dunn's mother Charlotte Charles told Sky News she had felt "sheer relief".

"I think that's the overwhelming feeling... just sheer relief that we could look up and say 'Harry, we've done it mate. We've done what we promised.'"