The family of a British teenager killed in a car crash with an American diplomat’s wife could launch a legal challenge.

Harry Dunn’s parents want the advice given by the Foreign and Commonwealth Office (FCO) to police over the decision to grant the suspect diplomatic immunity to be examined.

Anne Sacoolas is reportedly married to a US intelligence official and left the UK just days after the crash that killed the 19-year-old motorcyclist in August.

Northamptonshire Police force applied to waive her diplomatic immunity but was informed it had been declined after she had returned to the US.

But on Saturday Dominic Raab, the foreign secretary, wrote to the family telling them Ms Sacoolas no longer has immunity.

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Mr Dunn’s parents said they would resort to a judicial review if they were not satisfied by ministers’ answers over why Ms Sacoolas was allowed to leave the country.

The family’s spokesperson, Radd Seiger, said their lawyers were going to write to the FCO demanding records of the advice, including emails, messages and notes, given to Northamptonshire Police.

“What we don’t know is whether somebody cocked up or whether they were put under pressure by the Americans to concede,” he said.

“We want to conduct an investigation into the FCO’s decision to advise Northamptonshire Police that this lady had the benefit of diplomatic immunity.

“If we’re not satisfied, then we’ll go to a judicial review and ask a high court judge to review it all.”

Tim Dunn, the teenager’s father, argued Ms Sacoolas was “not entitled to immunity”.

Speaking at a press conference in New York on Monday, he said: “Somewhere, somebody has made a decision to give this lady immunity … they made a mistake, someone has made a mistake.”

Mr Raab met Mr Dunn’s parents last week but Mr Seiger accused the foreign secretary of being “cold” and “rude”.

Donald Trump defends US diplomat's wife who killed teenage Briton Harry Dunn in crash

“There were three lawyers in the room – why would you bring three lawyers into a room with a grieving mother? Why?” he said.

“There were 12 people in a room for a meeting that was supposed to be him expressing his condolences.

“Mr Raab was stiff, he was cold, he was unpleasant, he was rude. Then three days later we get another letter. That isn’t right.”

Following the meeting, a statement from the foreign secretary said he "shared the frustration of Harry’s mother and father".

He said Boris Johnson had spoken to Donald Trump about the case "and made clear that what has happened is not acceptable", but the US president later said driving on the wrong side of the road "can happen", adding: "It was an accident."

The collision happened outside RAF Croughton in Northamptonshire on 27 August.

Her Volvo collided with Mr Dunn on his Kawasaki motorcycle at around 8.30pm, and the teenager died a short time afterwards in hospital.

Ms Sacoolas’s lawyer, Amy Jeffress, told the Mirror her client was driving on the wrong side of the road and was sorry her mistake.

She has expressed a desire to meet Mr Dunn's family and apologise.

A family photo of Harry Dunn with his mother (PA)

Ms Jeffress said the diplomat’s wife flagged down help and spoke to Mr Dunn while he was injured, and added that her children were in the car.

Harry Dunn’s family said the teenager, from Banbury, “loved life” with his twin and siblings, and had a life-long passion for riding motorbikes.

“His love for his family and friends outshone everything and made him the caring and loving young man he was,” a statement said.

“The loss of Harry is going to leave the biggest hole in our lives and he will truly be missed by all.”

A spokesperson for Northamptonshire Police said the force “remains absolutely committed to getting Harry and his family justice”.

Superintendent Sarah Johnson said: “We are doing everything on our side to ensure that a full and thorough investigation, with the assistance of all parties involved, takes place, in order for this to be achieved.”

A spokesperson for the Foreign Office said the government was "continuing to press American authorities for the suspect’s cooperation" and working to support the family, police and Crown Prosecution Service.

"The individual concerned was covered by diplomatic immunity while she was in the UK under the Vienna Convention for Diplomatic Relations," a statement added.