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Boris Johnson has said the US has been "absolutely ruthless" in enforcing the code of diplomatic immunity after it emerged the suspect in the death of Harry Dunn would not return to the UK.

The Prime Minister has said that Donald Trump was sympathetic towards the 19-year-old's family on the immunity granted to Anne Sacoolas, reportedly married to a US intelligence officia;.

However, Mr Johnson said that the US are "very reluctant" to allow its citizens to be tried abroad and have been "absolutely ruthless" in enforcing its code of diplomatic immunity.

Harry, 19, died when his motorbike crashed into a car outside an RAF base on August 27.

Ms Sacoolas, who left the UK after the crash despite telling officers she did not plan to do so, is believed to have been driving on the wrong side of the road.

The teenager's family have now said they would be flying out to the US to "spread our campaign" and try and convince Mr Trump to reverse his decision on diplomatic immunity.

Commenting on the US's safeguarding of Mrs Sacoolas on Friday, the Prime Minister 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.

"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."

When asked about the diplomatic immunity row by reporters on Wednesday, Mr Trump called the incident a "terrible accident", but suggested driving on the opposite side of road was confusing - saying "it happens".

The family's lawyer and spokesman, Radd Seiger, said Harry's parents, Charlotte Charles and Tim Dunn, were engaging lawyers to take a civil case against Mrs Sacoolas in America.

In a post on the Justice 4 Harry GoFundMe page, the teenager's family said on Friday: "We are overwhelmed by the generosity and the support from all of you, and want to say thank you again personally from the bottom of our hearts.

"We couldn't do this without you and value each and everyone of you! The individual comments and personal stories that have been coming into us have touched our hearts and we know that we are not alone.

"We wanted to share with you that on Sunday we will be flying to the US to continue our fight for justice4harry.

"We will be meeting with various news channels to spread our campaign with all of America and ultimately hope that the American government see that it is right to send Anne Sacoolas back to the UK to face our justice system."