The US has indicated it will reject attempts to extradite Anne Sacoolas to the UK to face charges over Harry Dunn's death.
In a statement following the decision by the Crown Prosecution Service (CPS) to charge the diplomat's wife with death by dangerous driving, the State Department said returning her to the UK would be an "egregious abuse".
"The use of an extradition treaty to attempt to return the spouse of a former diplomat by force would establish an extraordinarily troubling precedent," the statement said.
The lawyer for the driver has said she "will not return voluntarily to the United Kingdom to face a potential jail sentence".
Charlotte Charles and Tim Dunn, the parents of 19-year-old Harry Dunn, broke down in tears after learning of the decision to bring charges.
Anne Sacoolas was involved in a crash in which Mr Dunn died outside an RAF base in Northamptonshire in August.
She returned to the US following the collision after claiming diplomatic immunity through her husband's work. He is thought to be a US intelligence officer.
Mrs Sacoolas, 42, has now been charged with causing death by dangerous driving. She was informed by her lawyer of the charges and that there will be an attempt to extradite her.
The family was one of the first to be informed by the director Max Hill that Mrs Sacoolas would be charged. The home secretary and foreign secretary were informed afterwards.
Speaking outside the CPS office, mother Charlotte Charles thanked those who had supported their case, including the media, and said: "We feel like we've taken a huge step in the start of achieving the promise [of justice] to Harry."
Tim Dunn, Harry's father, spoke briefly of his shock about the meeting and the decision to charge Mrs Sacoolas before being overcome by emotion.
Speaking to Sky News, Nick Adderly, the chief constable of Northamptonshire Police said: "We will support the Crown Prosecution Service where we can but we will now let justice run its course," adding that the police cannot now "interfere" in that process.
"We hope that we see Anne Sacoolas back in the UK to face justice."
Mr Adderly added, in what he described as "a plea", was that Mrs Sacoolas "has to live with this now for the rest of her life." He said "it may do her the world of good" for herself to "face the course of law and take whatever punishment they decide is appropriate.''
Sky's diplomatic editor, Dominic Waghorn says the state Department's response would seem to be at odds with the Foreign Office position.
Foreign Secretary Dominic Raab said immunity clearly ended when Mrs Sacoolas left the UK. The British position would appear to be therefore that neither she nor her husband enjoy diplomatic protection.
Following today's charging decision, Mr Raab said: "I welcome the taking of a charging decision which is an important step towards justice for Harry and towards solace for his family, but it is not the end.
"I hope that Anne Sacoolas will now realise the right thing to do is to come back to the UK and cooperate with the criminal justice process."
In the US State Department statement, the spokesperson added: "We express our deepest sympathies and offer condolences to the Dunn family for their loss. We will continue to look for options for moving forward.
"We are disappointed by today's announcement and fear that it will not bring a resolution closer."