The US has rejected a formal extradition request from the UK for Anne Sacoolas who left the country last year following a crash which killed 19-year-old Harry Dunn.
Sacoolas’s car hit motorcyclist Dunn outside RAF Croughton in Northamptonshire in August, which resulted in his death.
Sacoolas, 42, claimed diplomatic immunity – her husband is believed to be an American intelligence officer – and quickly fled the country.
In December, she was charged by the Crown Prosecution Service with death by dangerous driving.
“Following the Crown Prosecution Service’s charging decision, the Home Office has sent an extradition request to the United States for Anne Sacoolas on charges of causing death by dangerous driving,” a UK Home Office spokesman said in a statement on Friday.
“This is now a decision for the US authorities.”
However the US quickly responded by rejecting the request.
A state department spokesman said Washington would not grant the request.
“Under the circumstances of this case, we strongly believe that an extradition request would be highly inappropriate,” he said.
“The United States has been clear that, at the time the accident occurred, and for the duration of her stay in the UK, the driver in this case had status that conferred diplomatic immunities.
“The use of an extradition treaty to attempt to return the spouse of a former diplomat by force would establish an extraordinarily troubling precedent.”
It comes after Radd Seiger, a spokesman for the Dunn family, described the move as a “significant day”.
Seiger said he was “100 per cent sure that Anne Sacoolas will be back in the UK to face the justice system”.
“The precedent that she would set if she doesn’t come back would be unimaginable,” he said.
“I fundamentally believe as Harry did and Harry’s parents, that no one is above the law, whether you’re a diplomat or not, diplomatic immunity does not give you a get out of jail free card in these circumstances.”
Dunn’s family has claimed Sacoolas was driving on the wrong side of the road when she crashed into their son near the air force base used by the US military.
Sacoolas’s lawyer said she would not return to the UK voluntarily to potentially face jail for “a terrible but unintentional accident.”
Prime Minister Boris Johnson has said Sacoolas was wrong to use diplomatic immunity to leave the country and has urged US President Donald Trump to reconsider the country’s position.
Trump had hoped to persuade Dunn’s parents to meet Sacoolas when they visited the White House in October, but they declined.