The Home Office has described the US Secretary of State’s decision to reject an extradition request for the suspect charged in connection with the death of Harry Dunn as a “denial of justice”.
The 19-year-old’s parents were informed of Mike Pompeo’s decision in a phone call with their constituency MP, Andrea Leadsom, on Thursday.
In a statement, a Home Office spokeswoman said: “We are disappointed in this decision, which appears to be a denial of justice.
“We are urgently considering our options.”
Mr Dunn was killed when his motorbike crashed into a car outside a US military base in Northamptonshire on August 27 last year.
His family have said they are “not surprised” by the rejection, but added that it will “not be a battle the US government is going to win”.
In a statement, the US State Department said that at the time of the accident, and for the duration of her stay in the UK, the US citizen driver in this case had immunity from criminal jurisdiction.
A spokesman said if the US was to grant the UK’s extradition request, it would render the invocation of diplomatic immunity a practical nullity and would set an extraordinarily troubling precedent.
He described the US as having a history of close law enforcement co-operation with the UK, and said it values that relationship.
He expressed the State Department’s sincere condolences and sympathy to the Dunn family for the loss of their son.
Anne Sacoolas was charged with causing Mr Dunn’s death by dangerous driving, by the Crown Prosecution Service, in December.
The 42-year-old was granted diplomatic immunity following the crash and was able to return to her home country, sparking an international controversy.
The Home Office submitted an extradition request for her, which the US described as highly inappropriate.
Prime Minister Boris Johnson had previously said the chances of the suspect ever returning to the UK were very low.
Mr Dunn’s family said they would react fully to the news on Friday morning, but that “the fight goes on” for justice for their son.
The family’s spokesman, Radd Seiger, told BBC Radio 5 that he understood no reason had been given for Washington’s decision.
“The reality is they can’t give a reason because there is no good reason,” he said.
“It’s simply an unlawful act, and we knew it was coming.
“We’ll simply take it in our stride. Everybody’s relaxed, we knew this day was coming.”
Earlier, in a short statement, Mr Seiger said: “If Trump and Pompeo think this is an end to the matter, they have another think coming. Team Harry will sit down with the Government tomorrow and work out our next steps.
“And next steps there will be. The whole world is on Team Harry’s side. This is not a battle the US government is going to win.”
Mr Seiger also called for a meeting with Mr Johnson after what he described as “one of the darkest days in the history of this relationship”.
He told BBC Radio 4’s Today programme: “We did not welcome his public comments last week.
“He is the leader of the gang, he aspired to be Prime Minister. History was made last night when the Americans decided not to return her.
“Boris Johnson wanted to be Prime Minister, he is now being tested severely. I expect him today to rise to that challenge and come and meet with me and the family and tell us what he’s going to do about it.”
Mr Seiger added that the US administration was “taking a wrecking ball” to the relationship with the UK.
In a statement released on behalf of the suspect after she was charged in December, Mrs Sacoolas’s lawyers said: “Anne will not return voluntarily to the United Kingdom to face a potential jail sentence for what was a terrible but unintentional accident.”
Lawyers acting on behalf of the Dunn family have said it is the first time in the 100-year history of the extradition treaty that such a request has been turned down by the US.