Harry Dunn’s family have criticised the lawyer for the US woman charged over his death, saying her stance “boggles the mind and is deeply disturbing”.
They said the suggested reason for Anne Sacoolas not to return to the UK– a potentially long jail sentence – was “unworthy of someone of her standing”.
It comes after lawyer Amy Jeffress said the possible 14-year prison sentence was “not proportionate” for what was “a terrible but unintentional accident”.
Meanwhile, Boris Johnson has promised to continue to press the US on behalf of the family of Harry Dunn “at every level”.
Sacoolas, the wife of an American intelligence officer, left the country after the car she was driving was involved in a crash in which Mr Dunn died outside an RAF base in Northamptonshire in August.
The 42-year-old suspect sparked public outrage after claiming diplomatic immunity through her husband’s job.
It was only after she had left the UK on a military flight from the air base that the Foreign Office wrote to the 19-year-old’s family to say she did not have immunity.
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Sacoolas was charged with death by dangerous driving on Friday, following a decision by the Crown Prosecution Service.
But Ms Jeffress said her client had “co-operated fully with the investigation” and would not return voluntarily to the UK.
She said: “This was an accident, and a criminal prosecution with a potential penalty of 14 years’ imprisonment is simply not a proportionate response.”
The State Department has also said the extradition of a former diplomat’s wife to the UK would be an “egregious abuse”.
However, the Dunns said they took the comments with a “pinch of salt”.
Family spokesman Radd Seiger said the lawyer’s statement “boggles the mind and is deeply disturbing”.
He said: “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.”
He added: “But to seek to gainsay what penalty a judge might hand down, if any, before all the evidence is heard, and to use the maximum possible sentence, which is a matter entirely for the court having heard all the evidence and following clear guidelines laid down by legislators, as a reason not to return, frankly as I say, is unworthy of someone of her standing.
“She knows well that this is fair legal system, in fact one of the fairest in the world.”
The maximum sentence for death by dangerous driving is 14 years in prison but road safety charity Brake says that in most cases it is “much lower than the maximum, unless there are many aggravating factors and multiple deaths”.
Recent case law suggests that in the absence of such aggravating circumstances a sentence of one to two years would be more likely.
On a visit to Estonia to see British troops, Prime Minister Boris Johnson said the government was still trying to bring its pressure to bear.
“The law should take its course and we’ll be following the case with keen interest and continuing to make representations on behalf of Harry Dunn’s family in the US at every level,” said Mr Johnson.