Video report by ITV News Correspondent Neil Connery
Foreign Secretary Dominic Raab has contacted his US counterpart Mike Pompeo over the case of a diplomat's wife who claimed immunity after an alleged car crash.
It follows a collision in which 19-year-old Harry Dunn was killed after reportedly being crashed into by a car, allegedly being driven on the wrong side of the road by Anne Sacoolas.
Police want to speak to Ms Sacoolas over the crash, close to RAF Croughton, Northamptonshire, and have urged her to return to the UK to face investigation.
A spokesperson for the Foreign and Commonwealth Office said Mr Raab had "reiterated his disappointment with the US decision and urged them to reconsider".
It is thought Boris Johnson will personally contact the White House if the issue continues to be unresolved.
The prime minister said he hoped Mrs Sacoolas would "engage properly" with "the processes" of British laws.
"I do not think that it can be right to use the process of diplomatic immunity for this type for purpose."
"I hope Anne Sacoolas will come back and engage properly with the processes of laws as they're carried out in this country. And that's a point that we're raising today with the American ambassador in the UK and I hope it will be resolved very shortly.
"If we can't resolve it then of course, I will be raising it myself personally with the White House."
Police chiefs have already written to the US embassy in London to demand immunity is waived for Mrs Sacoolas.
Nick Adderley, chief constable for Northamptonshire Police, whose force is leading investigations into the August 27 crash, said US authorities had been appealed to in “the strongest terms” to apply a waiver and “allow the justice process to take place” in relation Mrs Sacoolas.
Police said the teenager died after his motorbike collided with a car close to RAF Croughton in Northamptonshire, a military base used by the US Air Force.
Superintendent Head of Operations Department Sarah Jonson said Northamptonshire Police were doing everything they could to support Harry's family and gather "all the evidence available", adding it was "imperative" the suspect return to the UK in order for the investigation to continue.
She told ITV News police officers had spoken to the driver on the day of the collision and again the following day in the presence of a legal advisor and a representative from the embassy.
The force's request for a waiver against diplomatic immunity was denied and they were told on September 16 the suspect had returned to the US.
Superintendent Jonson urged the suspect, who has not been named by police, to make contact with the force.
Speaking to ITV News, Superintendent Jonson said: "The collision occurred on the evening of 27 August and we were able to speak to the driver in company a legal advisor and a representative from the embassy on 28th."
She continued: "We were made aware after that meeting that diplomatic immunity was likely to be raised as an issue and applied immediately for a waiver in order for us to progress the investigation.
"That waiver is imperative in order for us to undertake an interview with the suspect in this case. We awaited a reply in terms of whether or not that waiver had been granted. On 16th September we were advised that the suspect had left the UK and the waiver had been declined."
The US embassy confirmed the incident had involved a vehicle driven by the spouse of a US diplomat assigned to UK who had departed the country, adding that diplomatic immunity was “rarely waived”.
Responding to a question on Twitter on Sunday, Mr Adderley confirmed that he and Stephen Mold, Police Fire and Crime Commissioner for Northamptonshire, had contacted the embassy for immunity to be waived.
Speaking to ITV News on Sunday, Mr Dunn’s parents said they had been “trapped in a living nightmare” since their son’s death.
His mother Charlotte Charles said they were prepared to travel to the US to seek a resolution to their situation.
“We’re not going to be swept under the carpet," Ms Charles said.
“Harry always fought for what he believed in… we’re going to carry on that.”
She added: “We’ll go as far as we need to go, to get justice for our boy and to do our best to stop another family suffering”.
The teenager’s father Tim Dunn said: “We can’t let our son die and then nothing be answered for.”
Under the Vienna Convention on Diplomatic Relations, families of diplomats are granted immunity from arrest or detention, with the sending state able to issue a waiver of that immunity.
According to the Crown Prosecution Service, the immunity does not apply to dependants of consular officials based outside of London.
However, it is understood that some diplomatic staff and their spouses located outside the capital can get that immunity.
Radd Seiger, a spokesperson for Mr Dunn’s family, said British authorities had asked their US counterparts for immunity to be waived “several times”.
He said: “They’ve been told the answer is no, we’ve learnt via the police. The answer has come back as no.”
Business Secretary and South Northamptonshire Tory MP Andrea Leadsom has already met with Mr Dunn’s family and said they are “totally heartbroken”.
She added: “We have to get proper justice for Harry and closure for his family.”
Mr Dunn’s family will meet with Foreign Secretary Dominic Raab in the coming days.