The US has said its decision to refuse an extradition request for Harry Dunn's alleged killer was final.
It comes after an Interpol Red Notice was issued for US national Anne Sacoolas who is now "wanted internationally".
A US official said she had diplomatic immunity, but Downing Street branded the refusal "a denial of justice".
Mr Dunn, 19, died after a crash in Northamptonshire, with Mrs Sacoolas accused of death by dangerous driving.
The shadow foreign secretary has accused the Foreign Office of "clear and repeated failings".
Mrs Sacoolas, the wife of a US intelligence official based at RAF Croughton, claimed diplomatic immunity following the crash and was able to return to her home country, sparking an international outcry.
The Interpol Red notice means she can be arrested if she leaves the US.
Mr Dunn's mother Charlotte Charles was notified by Northamptonshire Police about the Interpol notice.
She said it was "a huge step in the right direction".
Motorcyclist Mr Dunn died in a crash with a car near US military base RAF Croughton on 27 August.
A Home Office extradition request was refused by US secretary of state Mike Pompeo in January.
On Tuesday, the state department's spokeswoman said that decision was final.
She said that granting the extradition request for Mrs Sacoolas would have rendered the invocation of diplomatic immunity a practical nullity and would have set an "extraordinarily troubling precedent".
She added that the US has a history of close law enforcement co-operation with the UK, and values that relationship.
But the Prime Minister's official spokesman said Boris Johnson "has been clear that he wants to see justice served for Harry and his family".
Mr Johnson had raised the case with Donald Trump "on a number of occasions", the spokesman said.
"The US refusal to extradite Anne Sacoolas amounts to a denial of justice and she should return to the UK," the spokesman added.
Shadow foreign secretary Lisa Nandy has called for a parliamentary inquiry, in which the Foreign Secretary would have to explain "failings" with Mr Dunn's case.
But the Dunn family spokesman Radd Seiger said: "The White House may feel that secretary Pompeo's refusal to extradite Anne Sacoolas was final but that does not reflect the real position.
"In fact quite the contrary, as the US Embassy in London said in a recent letter to Andrea Leadsom, both countries recognise that the final decision will rest with the court following a judicial review."
An Interpol Red Notice is a request to law enforcement worldwide to locate and provisionally arrest a person pending extradition, surrender, or similar legal action.
On its website, Interpol states a red notice "is an international wanted persons notice, but it is not an arrest warrant".