The refusal of the United States to extradite a diplomat's wife charged over a car crash that killed a British teenager is a "denial of justice", the United Kingdom's foreign minister said on Friday.

The stern and unusual public rebuke comes in a case which has stirred up debates over the limits of diplomatic immunity.

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Anne Sacoolas has admitted to driving on the wrong side of the road on August 19 last year, when her car collided with 19-year-old Harry Dunn's motorbike, near an air force base in Croughton, central England, used by the US military as a communications hub.

"I called the US ambassador earlier to express the government's disappointment about this decision," said UK Foreign Secretary Dominic Raab in a statement emailed to Al Jazeera on Friday.

"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.

"I emphasised that work to improve road safety on and around the Croughton base must continue, and the need to resolve the issue whereby family members at RAF Croughton are immune from criminal prosecution."

It's one of the darkest days in the history of this special relationship. Boris Johnson wanted to be prime minister. He is now being tested severely. I expect him today to rise to that challenge. Dunn family spokesman

Lawyers acting on behalf of the Dunn family have said it is the first time in the 100-year history of extraditions between the UK and US that such a request has been turned down by the US.

Both countries currently adhere to the Extradition Treaty of 2003, which came into force in 2007. In the terms of the treaty, a person may only be extradited if the offence they are charged with would result in a custodial sentence of one year or more.

The Dunn family's spokesman lashed out at Prime Minister Boris Johnson on Friday morning.

"He is the leader of the gang," he told BBC Radio 4. "He aspired to be prime minister. History was made last night when the Americans decided not to return ... It's one of the darkest days in the history of this special relationship.

"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."

Business Secretary Andrea Leadsom, the constituency minister of parliament for Harry Dunn's family, agreed that 42-year-old Sacoolas "should come back to the UK".

"I am deeply sorry that extradition has been refused," Leadsome posted on Twitter. "This was a tragic road accident where a much loved young man died. His family are heartbroken. The person who has been charged by the CPS [Crown Prosecution Service] should come back to the UK #Justiceforharry"

I am deeply sorry that extradition has been refused. This was a tragic road accident where a much loved young man died. His family are heartbroken. The person who has been charged by the CPS should come back to the U.K. #Justiceforharry — Andrea Leadsom MP (@andrealeadsom) January 24, 2020

Leadsom met with US ambassador Woody Johnson, the commander of RAF Croughton Colonel Bridget McNamara and the Chief Constable of Northamptonshire Police on Friday morning at an undisclosed location.

The case has been a thorn in London's close relations with Washington, and a political headache for Johnson, who is attempting to cultivate trade relations in a bid to offset the potential damage of Britain's withdrawal from the European Union.

Diplomatic shambles

Sacoolas was charged by British police with causing death by dangerous driving, but cited immunity and refused to return to the UK to face justice, as Dunn's parents have demanded.

Britain's Home Office submitted an extradition request for Anne Sacoolas on January 10, which was rejected by US Secretary of State Mike Pompeo on Thursday, with Pompeo agreeing Sacoolas had immunity from criminal jurisdiction during her stay in the UK.

Floral tributes lay on the roadside at the spot where Harry Dunn was killed [File: Lindsey Parnaby/AFP]

"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 State Department said.

A State Department spokesman described the US as having a history of close law enforcement cooperation with the UK and said it valued that relationship. He expressed the department's sincere condolences and sympathy to the Dunn family for the loss of their son.

Limited options

Dunn's parents were informed of the decision in a phone call with their minister of parliament on Thursday and "were not at all surprised", said family spokesman Radd Seiger.

"This is a lawless, corrupt administration that appears intent on attacking even its closest international ally," he said of the White House.

"If Trump and Pompeo think this is an end to the matter, they have another thing coming to them," he said, saying the family would meet British government representatives to discuss their next steps.

But there are a limited number of options available to the family.

The first, a process which has already begun, is to pursue a civil claim against Sacoolas in the US. They have previously said the claim could be worth "millions of pounds".

If Trump's administration is voted out in the next election, a new government may take a different approach to the extradition request - which the family say "will not go away".

Alternatively, Sacoolas could waive the extradition and surrender herself to the UK authorities. However, lawyers acting on her behalf have said she will not return to the UK

Dunn's parents visited the White House in October to meet US President Donald Trump.

They said he was warm and welcoming - but they criticised the White House's attempts to engineer a snap meeting with Sacoolas, who was in a room next door with press photographers.

Trump has called the crash a "terrible accident", saying it was common for Americans in the UK to have difficulty driving on the left side of the road.