The family of 19-year-old Harry Dunn has said the Foreign and Commonwealth Office (FCO) has written to them to say a US suspect in the case does not have diplomatic immunity.
The teenager died when his motorbike crashed into a car outside RAF Croughton in Northamptonshire on August 27.
The suspect, 42-year-old Anne Sacoolas – who is reportedly married to a US intelligence official – was granted diplomatic immunity following the crash.
But the family’s spokesman Radd Seiger said the Foreign Secretary Dominic Raab has written to Mr Dunn’s family about Mrs Sacoolas, saying: “The US have now informed us that they too consider that immunity is no longer pertinent.”
The letter, sent by Dominic Raab to the family, said: “We have pressed strongly for a waiver of immunity, so that justice can be done… Whilst the US government has steadfastly declined to give that waiver, that is not the end of the matter.
“We have looked at this very carefully… 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.”
Mr Raab added that the matter was now “in the hands” of Northamptonshire Police and the CPS.
An FCO spokesman told the PA news agency that the office “would not be commenting further on the content of the letter”.
Before the letter was sent by the FCO, the family’s lawyer Mark Stephens told PA: “There are approximately 20,000 official diplomats in this country – there’s a definitive list of who is and who isn’t.
“We know definitively that this guy was not a diplomat and therefore was not entitled to diplomatic immunity. That has a number of consequences.
“That means that the Americans have made a false claim. She would not have been entitled to claim diplomatic immunity.”
Meanwhile, Mrs Sacoolas’s legal representative Amy Jeffress, from the law firm Arnold and Porter, said: “Anne is devastated by this tragic accident.
“No loss compares to the death of a child and Anne extends her deepest sympathy to Harry Dunn’s family.”
Mr Seiger said in a statement that he had spoken with Ms Jeffress and the pair had agreed “to get together asap… to discuss how we are going to achieve a solution”.
He added that he was studying the FCO letter “with legal and political experts” to “fully understand where that leaves us”.
He added: “That all said, clearly a positive step forward but we won’t rest until we get justice for Harry.”
It comes as Mr Dunn’s parents Charlotte Charles and Tim Dunn flew to the US on Sunday to “put pressure on the US administration to do the right thing”.
Speaking to BBC News while on a flight to the US, Ms Charles said: “The statement from (Mrs Sacoolas’s) lawyer is promising, that we may be able to hopefully get a meeting put together.
“Whether it’s face-to-face or lawyer-to-lawyer, we’re not really sure yet, but fingers crossed we’re stepping in the right direction.”
Earlier, Mrs Sacoolas’s lawyer said: “Anne would like to meet with Mr Dunn’s parents so that she can express her deepest sympathies and apologies for this tragic accident.
“We have been in contact with the family’s attorneys and look forward to hearing from them.”
The lawyer said Mrs Sacoolas spoke with “authorities” at the scene of the crash and met Northampton police at her home the following day.
“She will continue to cooperate with the investigation,” they added.
On Friday, the Prime Minister said America was “absolutely ruthless” in its safeguarding of Mrs Sacoolas following the decision to grant her diplomatic immunity.
Boris Johnson said although President Donald Trump was sympathetic towards Mr Dunn’s family’s views on the use of diplomatic immunity, the US is “very reluctant” to allow citizens to be tried abroad.
Speaking of taking their campaign to the US, Mr Dunn’s family said in a statement that they “continue to live in a nightmare” and have so far been unable to grieve after his death.
Mr Dunn’s family are due to fly to the US on Sunday.
A statement released on behalf of the family said: “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 and return the driver Mrs Sacoolas to England to face the consequences of her actions.”