Home Secretary Priti Patel has formally sent an extradition notice to the US Justice Department for Anne Sacoolas over a fatal accident in which a British teen was killed.
Mrs Sacoolas left the UK after the car she was driving hit Harry Dunn, 19, outside RAF Croughton in Northamptonshire in August.
In December, she was charged by the CPS with death by dangerous driving.
She is still in the US having claimed diplomatic immunity shortly after the fatal accident. Her husband is thought to be a US intelligence officer.
Radd Seiger, a spokesman for the Dunn family, described the move as a “significant day”.
He told Sky News: “Everything they have been through it’s a very, very significant day toward the promise that they made to their son the night that he died, which is they would get him justice.
“We are much, much closer now than we were a few months ago when they were told nobody would be held accountable.
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“I am 100% sure that Anne Sacoolas will be back in the UK to face the justice system. Anne Sacoolas has to come back and she will come back, I have no doubt.
“The precedent that she would set if she doesn’t come back would be unimaginable.”
He added: “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.”
The US State Department and Mrs Sacoolas’ legal team have both indicated she will not willingly return to face the charge.
Last month, a lawyer for Mrs Sacoolas suggested a potentially long jail sentence over the accident was “unworthy of someone of her standing”.
Amy Jeffress said the possible 14-year prison sentence was “not a proportionate response” for what was “a terrible but unintentional accident”, while the State Department said the extradition of a former diplomat’s wife would be an “egregious abuse”.
A spokesman for the Home Office said: “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.
“This is now a decision for the US authorities.”