Harry Dunn’s family say the special relationship has seen “one of the darkest days” after the US rejected an extradition request for his alleged killer.
The 19-year-old motorcyclist died following a crash with diplomat’s wife Anne Sacoolas, 42, outside a US intel base on British soil.
Earlier this month the Home Office applied for the mum-of-three to be extradited to face a charge of death by dangerous driving.
But it has now been announced that US Secretary of State Mike Pompeo has turned down the request.
Harry’s furious family say they remain determined to win justice, even if that means waiting for the “corrupt” Trump administration to fall from power.
Family spokesman Radd Seiger said: “That’s the first time in history that the United States has turned down an extradition request.
“It’s one of the darkest days in the history of this special relationship.
“It’s a completely indefensible decision both legally and morally.
“The reality is that this administration, which we say is behaving lawlessly and taking a wrecking ball to one of the greatest alliances in the world, won’t be around forever whereas that extradition request will be.
“We will simply plot and plan for a reasonable administration to come in one day and to reverse this decision.”
Harry died last August after a head-on crash with Sacoolas near RAF Croughton, Northants.
The base has been loaned to the US for decades and it is understood her husband worked there.
Sacoolas initially cooperated with police but left the country weeks later citing diplomatic immunity.
Harry’s parents Charlotte Charles, 44, and Tim Dunn, 50, travelled to America in a bid to persuade her to return to the UK.
But despite meeting Trump in the White House they were unsuccessful.
Last week Prime Minister Boris Johnson described the chances of Sacoolas ever returning to the UK as “very low”.
The US State Department announced their rejection late on Thursday night.
A spokesperson said: “If the United States were to grant the UK’s extradition request, it would render the invocation of diplomatic immunity a practical nullity and would set an extraordinarily troubling precedent.”
The Dunn family were informed of the decision by their constituency MP Andrea Leadsom, who was due to meet US ambassador Woody Johnson in London yesterday.
When asked if PM Johnson is doing enough to bring Sacoolas in front of the courts, Mr Seiger added: “I have to say at the moment not.
“Boris Johnson wanted to be Prime Minister, he is now being tested severely.
“I expect him today to rise to that challenge and come and meet with me and the family and tell us what he’s going to do about it.
“...The whole world is on Team Harry’s side. This is not a battle the US Government is going to win.”
Yesterday Johnson’s spokesman said he was “disappointed” by the decision but admitted the PM had no plans to meet Harry’s family.
Foreign Secretary Dominic Raab added: “I called the US Ambassador earlier to express the Government’s disappointment about this decision.
“We feel this amounts to a denial of justice, and we believe Anne Sacoolas should return to the UK. We are now urgently considering our options.
“I also explained that the UK would have acted differently if this had been a UK diplomat serving in the US.”