The US diplomat’s wife allegedly involved in a crash which killed a teenager does not have diplomatic immunity, the Foreign Office has said.
A letter, that appears to have been sent by foreign secretary Dominic Raab to Harry Dunn’s parents, Charlotte Charles and Tim Dunn, says: “The question remains when such immunity comes to an end, regardless of any waiver.
“The UK government’s position is that immunity, and therefore any question of waiver, is no longer relevant in Mrs Sacoolas’ case, because she has returned home.
“The US have now informed us that they too consider that immunity is no longer pertinent.
“In these circumstances, Harry’s case is now a matter for Northants police and the CPS to take forward.”
The letter came after the lawyer of Anne Sacoolas, whose husband is reportedly a US intelligence official, said she was “devastated” by what happened to Harry.
Harry, 19, died when his motorbike crashed into a car, allegedly driven by Ms Sacoolas, outside an RAF base in Northamptonshire on 27 August.
His death has become the subject of a dispute between London and Washington after Ms Sacoolas returned to the US.
Ms Sacoolas’ legal representatives contacted the lawyer representing Harry's parents on Saturday just as they prepared to fly to the US on Sunday to drum up support.
Radd Seiger, spokesman and adviser for the family, told ITV on Saturday: “I have just landed on a flight from London and had a very brief phone conversation with the legal team representing Anne Sacoolas.
“We have agreed to meet each other at the earliest possibility as soon as we can co-ordinate our diaries.”
Harry’s parents plan to visit New York and Washington DC to “put pressure on the US administration to do the right thing”.
They have also appealed for any witnesses who saw Ms Sacoolas leaving the UK to get in touch.
Mr Seiger said: “I would urge anyone, on either side of the Atlantic, who has any information relating to Ms Sacoolas’ return to the United States, whether before, during, or after her departure, to please come forward.”
The UK government has sought a waiver of diplomatic immunity in the case, with Boris Johnson calling the US “absolutely ruthless” in its safeguarding of Ms Sacoolas.
Under the 1961 Vienna Convention, foreign diplomats and their families are immune from prosecution in their host country, meaning they cannot automatically be arrested or charged for a crime.
However, the Foreign Office can ask a foreign government to waive immunity in certain circumstances.
After speaking to Donald Trump, Mr Johnson said the US president was sympathetic towards Harry’s family on the issue of diplomatic immunity, but that the US was “very reluctant” to allow one of its citizens to be tried abroad.
Harry’s family have said they “continue to live in a nightmare” and have so far been unable to grieve after his death.
“As if losing Harry was not enough, they now find themselves having to expend enormous time and energy, which they can ill afford, generating sufficient publicity to garner public support to persuade the US government to help achieve closure,” a statement released on behalf of the family said.
“Sadly, all such efforts to date have failed.”
The family want Ms Sacoolas to return to the UK to “face the consequences of her actions”.
Ms Charles and Mr Dunn will meet with US media and politicians to ask for support in their son’s case.
“They are looking forward to making as many new friends in the USA as possible,” the statement added.
“Harry’s family will simply not leave matters where they are and will do whatever it takes, including taking legal action if necessary, to secure justice for Harry.”
Agencies contributed to this report.