Speaking on the fourth anniversary of the 19-year-old's death, Charlotte Charles said the US government had "very much misbehaved".

She told Sky News: "So did our government, but we got there in the end."

Despite it being a "long, laborious process", she did not want to let Harry or his twin brother down.

She added: "And I also felt I'd be letting the country down because without our fight that we took on, it would have kept happening to other families, and we didn't want that."

Harry was riding his motorbike on 27 August 2019 when he collided with a car outside RAF Croughton in Northamptonshire.

At the wheel, and on the wrong side of the road, was Anne Sacoolas, initially described as a "diplomat's wife".

At the time of the crash she was actually employed by a US intelligence agency, it later transpired.

She left the UK the same month, claiming diplomatic immunity.

Getting justice for Harry should have been "straightforward", Mrs Charles said.

It didn't turn out that way, but "when you make a promise to your child" there is a "fire that burns in the pit of your stomach", she continued.

Harry had already died when she got to the hospital, his body so badly injured she was told "not to touch certain areas of him".

But his face, because he had an "amazing crash helmet", was "pretty much untouched".

She kissed his forehead and, in front of family and friends, made a promise that "justice would be done".

Anne Sacoolas eventually pleaded guilty to causing Harry's death by careless driving and was sentenced to eight months in prison, suspended for 12 months, last December.

She was not at the Old Bailey in person, however, appearing via video link from Washington.

The court heard she had been advised by American officials not to fly to the UK, as her return "could place significant US interests at risk". The judge was critical of Mrs Sacoolas's non-attendance.

The case was a "a straightforward head on road collision", Mrs Charles said.

"It didn't turn out the way we expected it to because the US government very much misbehaved," she told Sky News.

Mrs Charles says she feels calmer now, and, along with other members of the family, will mark the fourth anniversary of Harry's death in Portland, Weymouth, where they scattered his ashes in 2020.

"Last year I felt that I was still letting Harry down in a sense because I hadn't achieved the promise that I'd made to him on the night that he died that I would get him justice.

"Now that justice was done, back in December last year, I feel that I can smile more this year."