The US has turned down an extradition request for a woman who is to be charged with causing the death of teenage motorcyclist Harry Dunn.
Mr Dunn, 19, died after a crash in Northamptonshire in August which led to the suspect Anne Sacoolas, the wife of a US intelligence officer, leaving for the US under diplomatic immunity.
Family spokesman Radd Seiger said they had taken the news “in our stride”.
The Home Office said the decision appeared “to be a denial of justice”.
Extradition proceedings were launched earlier this month.
Speaking to BBC Radio 4’s Today programme, Mr Seiger said the latest move had been “factored it into our planning and strategy”.
“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, they won’t be around forever whereas that extradition request will be,” he added.
“We will simply plot and plan for a reasonable administration to come in one day and to reverse this decision.”
The US Secretary of State Mike Pompeo turned down the extradition request in an email to the UK Government on Thursday evening.
Washington said granting the request would “render the invocation of diplomatic immunity a practical nullity”.
The family’s constituency MP Andrea Leadsom is due to meet the US ambassador Woody Johnson in London later to discuss the case.
Prime Minister Boris Johnson previously said the chance of Ms Sacoolas, who is to be charged with causing the death by dangerous driving, ever returning to the UK was very low.
Mr Seiger said “no reason” was given by Mr Pompeo in rejecting the extradition request.
“It’s one of the darkest days in the history of this special relationship,” he said.
“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.”
Mr Dunn died after his motorbike was in collision with a car owned by Mrs Sacoolas.
The crash happened outside RAF Croughton where Mrs Sacoolas’ husband Jonathan worked as an intelligence officer.
The 42-year-old left the UK and returned to her native US, claiming diplomatic immunity.
In a statement released on behalf of the suspect after she was charged in December, Mrs Sacoolas’s lawyers said: “Anne will not return voluntarily to the United Kingdom to face a potential jail sentence for what was a terrible but unintentional accident.”
The Home Office said it was “disappointed in this decision which appears to be a denial of justice”.
“We are urgently considering our options,” a spokeswoman added.
A statement from the US State Department said: “At the time the accident occurred, and for the duration of her stay in the UK, the US citizen driver in this case had immunity from criminal jurisdiction.
“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.”