Harry Dunn's family have hit back at the "mind-boggling" claims made by the lawyer of the woman suspected of killing their son, after they claimed she had "co-operated fully" with the investigation.

The Dunn family have urged Anne Sacoolas, the wife of a US diplomat, to return to the UK after she was charged with causing death by dangerous driving.

The incident happened in August this year near the RAF Croughton base in Northamptonshire, where her husband had been stationed, but Sacoolas's lawyers said she will not return to the UK to face charges.

Dunn family spokesman Radd Seiger said: “I know (Ms Jeffress) to be one of the finest and most outstanding lawyers in the USA. Her statement however boggles the mind and is deeply disturbing.

“For Ms Jeffress to seek to undermine one of the most mature, well-developed legal systems in the world, which has fairness at its heart, and which many countries around the world have modelled their legal systems on, is unbecoming of any lawyer, let alone someone of her stature.”

Mr Seiger urged Sacoolas to “put that defence forward in court here rather than ventilate it publicly”.

He added: “Like everyone else (in the UK) she will get a fair trial.”

A statement from Amy Jeffress, Sacoolas’s lawyer, said she had “co-operated fully with the investigation”.

She added: “Anne will not return voluntarily to the United Kingdom to face a potential jail sentence for what was a terrible but unintentional accident.”

Anne Sacoolas has said she will not willingly return to the UK. Credit: Handout

Prosecutors have begun the extradition process to bring her back to the UK, a decision the US said was “disappointing” and “unhelpful”.

It was only after she left the UK on a military flight directly from the air base that the Foreign Office wrote to the family to say immunity in her case was not valid.

The US government has maintained that Sacoolas had diplomatic immunity at the time of the incident.

It added: “It is the position of the United States government that a request to extradite an individual under these circumstances would be an egregious abuse."

Asked during a visit to Estonia if suspect Anne Sacoolas should be extradited to the UK, Mr Johnson said: “I think the best thing that I can say there is that the law should take its course and we will be obviously following that case with keen interest and continuing to make representations on behalf of Harry Dunn’s family at every level.”