LONDON (AFP) - The family of a British teenage motorcyclist killed in a crash with a US diplomat's wife have reached agreement on a civil claim, their spokesman said on Tuesday (Sept 21).

Harry Dunn, 19, died when his motorcycle collided with a car driven by Anne Sacoolas near a US military base in Northamptonshire, central England, in August 2019.

The case has dogged US-British relations after Sacoolas fled the country soon afterwards, claiming diplomatic immunity from prosecution, and US refusals to extradite her.

Dunn's family launched a case in the US state of Virginia, seeking damages against Sacoolas, who has acknowledged she was driving on the wrong side of the road, and her husband.

Radd Seiger, a spokesman for Dunn's parents, Charlotte Charles and Tim Dunn, said agreement had been "reached successfully between the parties".

He told Britain's domestic Press Association news agency the resolution came as "some considerable relief", without disclosing details.

"It is never easy mounting a legal battle for justice abroad, let alone in the USA, but the family's courage and determination to see this through has been incredible," he added.

In July, Britain said US President Joe Biden - whose first wife and daughter were killed in a car crash in 1972 - had agreed in principle to a remote trial for Sacoolas.

But Washington indicated it would not agree to her extradition, amid reports that Sacoolas and her husband, a technical assistant at the air base, worked in intelligence.

Seiger said the Dunn family would now seek to focus on an upcoming British coroner's inquest and ongoing criminal proceedings where they are looking to overturn a previous court ruling on Sacoolas' diplomatic immunity.

"Harry's family will never be able to move on from his loss, but they are more determined than ever to continue to move forward," he said, as he called for a parliamentary inquiry into how the case had been handled.

"This is a pivotal point in the campaign, a real milestone. But there is much work left to be done before justice for Harry can be said to be done," Seiger added.