The “special relationship” between the UK and the US is being severely tested by the case of a 42-year-old American woman, Anne Sacoolas, who fled Britain invoking diplomatic immunity after knocking down and killing Harry Dunn, a 19-year-old motorcyclist, while apparently driving on the wrong side of the road.

President Donald Trump has indicated he will not send Sacoolas, the wife of an US intelligence official, back to the UK to face trial for causing “death by dangerous driving” — this can carry a custodial sentence — despite an appeal from the British Prime Minister Boris Johnson.

Aware the case has outraged public opinion in Britain, Boris telephoned Trump to ask for diplomatic immunity to be waived so that Sacoolas could be brought back to face justice in the UK but the US President while acknowledging there had been a “terrible accident”, said Americans were confused by the British driving on the left and that “it happens”.

Boris, who hopes to do a post-Brexit trade deal with the US, said: “In my experience, America is very, very reluctant to allow its nationals to be tried overseas, and is absolutely ruthless in enforcing the code of diplomatic immunity.”

He added: “I must say, I don’t think it was appropriate for that provision to be used in this case. I made that point to the President, and he is sympathetic. I think we’ve just got to keep working on that and see what we can do to get justice for Harry Dunn and his family.”The accident happened on August 27 when Harry was knocked off his motorbike by Sacoolas outside the Royal Air Force base in Croughton, a US intelligence hub in Northamptonshire.

Sacoolas, who had only been in the UK for three weeks, had her two children in the car with her at the time. She was spirited out of the country by the US embassy on a special flight.

Anne Sacoolas. (Credit: Facebook)

In 2016, a typical year, there were 181,384 accident casualties recorded on Britain’s roads, of which 1,792 of these were fatal.

But Harry’s death has now developed into a national and international crisis, with his parents, Charlotte Charles and Tim Dunn, set to fly to the US next week to see if they prick America’s conscience.

The Daily Mail, which has turned Harry’s death into a campaign aimed at securing Sacoolas’s extradition, has interviewed the victim’s mother.

“Two days ago, Harry Dunn’s mother Charlotte finally forced herself to strip her 19-year-old son’s bed,” the paper reported. “He last slept in it six-and-a-half weeks ago, the night before he was killed in a head-on collision.

“ ‘That broke me, really broke me,’ says Charlotte. ‘It was the first time in six weeks I’ve really cried — really, really cried.

There have been little breakdowns but we’ve all had to try to be stoic for Niall.’

“Niall is Harry’s non-identical twin. He is devastated by his brother’s death.”

Harry’s parents lobbied the foreign secretary Dominic Raab but emerged frustrated from the meeting.

“ ‘This is an abuse of diplomatic immunity,’ says Charlotte. ‘When the laws were drawn up in the Sixties they weren’t intended to protect a diplomat’s wife who had, albeit unintentionally, killed a 19-year-old boy in an accident. No one else should ever have to suffer as we are — as Harry did.’ ”

Charlotte, 44, who works in a doctor’s surgery, has given the Mail a heartrending account of every parent’s nightmare – going to hospital to view her son’s body.

“When Charlotte sat beside her son’s terribly broken body at Oxford’s John Radcliffe Hospital six-and-a-half weeks ago she made him a promise. ‘We’d asked if we could do organ donation,’ she says. ‘They just looked at us and said, ‘I’m sorry. There’s nothing to donate.’ There wasn’t anything much that wasn’t broken either inside or out.

“ ‘We were only left with tissue donation. I believe that’s the retinas of his eyes and the skin that wasn’t touched.’ ”