The Crown Prosecution Service (CPS) has told Harry Dunn’s parents it will continue to pursue the prosecution of their son’s alleged killer, despite the High Court ruling she had diplomatic immunity.

Anne Sacoolas was charged with causing the teenager’s death by dangerous driving 12 months ago after a fatal road crash outside a US military base in Northamptonshire on August 27 last year.

Chief Crown Prosecutor for the East Midlands, Janine Smith, wrote to the 19-year-old motorcyclist’s family just before the anniversary of the CPS’s charging decision, to say she was “satisfied that there remains sufficient evidence for a realistic prospect of conviction.”

Sacoolas, 43, was able to return to her home country after the US Government asserted diplomatic immunity on her behalf, sparking an international controversy.

Despite the charging decision by the CPS, and an extradition request submitted by the Home Office, the US State Department refused to return her to the UK – describing the decision as “final”.

In a meeting with the Director of Public Prosecutions in September, the family was told the CPS had concluded Sacoolas did not have immunity at the time of the collision.

The family had contested the diplomatic immunity asserted on behalf of the suspect, but High Court judges accepted the Foreign Office’s position that Sacoolas “enjoyed immunity from UK criminal jurisdiction”.

Mr Dunn’s parents, Charlotte Charles and Tim Dunn, have been granted permission to appeal against the judgment.

In Ms Smith’s letter to Mrs Charles and Mr Dunn, seen by the PA news agency, she said: “I wanted to take the opportunity to provide an update following our consideration of the recent High Court judgment.

“Having considered the judgment, and notwithstanding the outcome in respect of diplomatic immunity, I am satisfied that there remains sufficient evidence for a realistic prospect of conviction and that it remains in the public interest for the prosecution to continue.

“The CPS remains of the view that Mrs Sacoolas should return to the UK to stand trial.

“I do hope this confirmation is of some small comfort to Harry’s family and friends at this challenging time of the year.”

Mrs Charles said the family would be “nowhere” without the CPS – adding that she was “really pleased” with Ms Smith’s letter.

She told PA: “My family and I are really pleased to see this letter from the CPS.

“We are approaching our second Christmas without Harry and without justice for Harry.

“Our pain is raw and real and we need to get this done sooner rather than later.

“In our darkest hour we know we can continue to count on the CPS to continue to do their job. We would be nowhere without them.”

The family’s spokesman, Radd Seiger, told PA he believed the letter showed the “CPS remain of the view” that Anne Sacoolas was not entitled to diplomatic immunity.

He said: “This is an important letter from the Chief Crown Prosecutor.

“It signals yet again that the CPS remain of the view, as do Harry’s parents on legal advice, that Anne Sacoolas did not have diplomatic immunity at the time of the crash and intend to proceed with their prosecution of her notwithstanding the judgment of the High Court.

“We have always been clear on that issue.

“We look forward to holding talks with both governments in the new year to bring the family’s torture to an end.”