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.
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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 were 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 can not now "interfere" in that process.