The US Government has maintained its position on the diplomatic immunity granted to Harry Dunn’s alleged killer, despite a letter from the teenager’s parents urging them to change their minds.

Downing Street backed the letter, addressed to President Donald Trump and US Secretary of State Mike Pompeo, which asked the pair to review the decision to block an extradition request for Anne Sacoolas.

The letter, from Charlotte Charles and Tim Dunn, came after the PA news agency reported that lawyers acting on behalf of the 42-year-old suspect had opened up a dialogue with the Foreign Office earlier this year to work towards a “resolution”.

Mrs Charles and Mr Dunn urged the president and the secretary of state to reconsider the decision to refuse the Home Office’s request to extradite Mrs Sacoolas, saying that the UK justice system has “fairness at its heart”.

But a spokeswoman for the State Department reiterated its position that at the time of the accident, and for the duration of her time in the UK, the driver had immunity from criminal jurisdiction.

She told PA that the Foreign Secretary had made that clear in Parliament on October 21, 2019 and since the accident occurred, the United States had been closely engaged with the UK government, and have been transparent about their positions on legal and diplomatic matters.

But the Prime Minister’s official spokesman said they backed the family’s efforts to lobby the US president.

Asked if the UK backed the letter sent by the Dunn family, he said: “Yes, we are very clear that we want Harry’s family to get the justice they deserve.”

Ms Charles and Mr Dunn met Mr Trump at the White House six months ago, where he had hoped they would meet the suspect, who was waiting in an adjacent room, but they declined to see her.

They took their campaign for justice to the US in October, two months after their 19-year-old son was killed when his motorbike collided with a car outside a US military base in Northamptonshire.

Mrs Sacoolas, the wife of a US intelligence official based at RAF Croughton, claimed diplomatic immunity following the crash and was able to return to her home country, sparking an international controversy.

She was charged with causing the death by dangerous driving of Mr Dunn in December but an extradition request was rejected by Mr Pompeo in January.

The Foreign Office has since come under fire after documents revealed that a senior diplomat sent a text message to their US Embassy counterpart saying there was “not much mileage” in Mrs Sacoolas and her family staying in the UK, adding: “I think you should feel able to put them on the next flight out.”

Responding to the criticism, the Prime Minister’s official spokesman said: “The Foreign Office have set out, from the outset, that they believe Harry’s family deserve justice and they have been working hard to help them secure it.”

Urging Mr Trump to reconsider the decision to refuse the extradition request, the teenager’s parents said: “It is now six months since we met with you, Mr President, in the Oval Office. We hope you are as well as can be in the current difficult circumstances.

“When we parted last October, you kindly mentioned that you would take a different look at the case and we have not heard from you since.

“Of course, since then, the UK have sought Mrs Sacoolas’ extradition, which has been rejected by Secretary Pompeo.

“We note from the British press that Mrs Sacoolas’ lawyer has reached out to the Foreign Office here in London and that she is attempting to seek a resolution to the problem. We understand that Mrs Sacoolas must be suffering too.

“The purpose of this letter is to ask you please to review your decision to not allow her to return to face the justice system, which, as you know, has fairness at its heart.

“As we are sure you can appreciate, it is really the only way forward in the interests of both families and of relations between our two nations, which have suffered as a result of the decision.

“If you allow her to return, we are sure only good can come from it. Otherwise, the problem will never go away. No-one is above the law.”

The State Department spokeswoman said the US Government again expresses its sincere condolences and sympathy to the Dunn family for the loss of their son.