The family of Harry Dunn, the British teenager killed in a hit-and-run, has become embroiled in a war of words with the lawyer representing the US intelligence official’s wife allegedly responsible for the fatal crash.

After months of campaigning, Harry’s parents broke down in tears on Friday when the Crown Prosecution Service (CPS) announced it was charging Anne Sacoolas – who fled back to America claiming diplomatic immunity following the incident – with death by dangerous driving.

But a statement released by Sacoolas’s lawyer, which made clear she would not return voluntarily to the UK and claimed her potential 14-year sentence for the offence was disproportionate, has been condemned by Dunn’s parents’ spokesman, who labelled it “deeply disturbing”.

After the CPS announced its decision to charge and begin extradition proceedings, Sacoolas’s lawyer, Amy Jeffress, reacted by saying her client was devastated by the “tragic accident”. She said: “Anne would do whatever she could to bring Harry back. She is a mother herself and cannot imagine the pain of the loss of a child. She has cooperated fully with the investigation and accepted responsibility … This was an accident, and a criminal prosecution with a potential penalty of 14 years’ imprisonment is simply not a proportionate response.”

Though Jeffress said they had been speaking with UK authorities, she stressed: “Anne will not return voluntarily to the United Kingdom to face a potential jail sentence for what was a terrible but unintentional accident.”

On Saturday, Radd Seiger – who is a spokesman for Harry’s parents, Tim Dunn and Charlotte Charles – hit out at the lawyer’s comments. “We have also seen the statement issued by Amy Jeffress, who I know to be one of the finest and most outstanding lawyers in the USA. Her statement however 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.

“To the extent that Ms Jeffress feels her client has a defence to the charge, I would urge on behalf of the family, for Mrs Sacoolas to put that defence forward in court here rather than ventilate it publicly. Like everyone else here, she will get a fair trial.

“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.”

Harry, 19, died outside RAF Croughton, in Northamptonshire, when his motorbike was involved in a crash with a car allegedly driven by Sacoolas on the wrong side of the road. Harry’s parents have been fighting a legal battle on both sides of the Atlantic to force Sacoolas to come back to the UK to face the charges.

Play Video 0:54 Harry Dunn's mother speaks after Anne Sacoolas is charged over son's death – video

Responding to the CPS charging decision on Friday, the US Department of State said the statement was “disappointing and will not bring a resolution closer”. It reiterated that it believed Sacoolas had diplomatic immunity, with a spokesman adding: “The use of an extradition treaty to attempt to return the spouse of a former diplomat by force would establish an extraordinarily troubling precedent.”

In his statement on Saturday, the Dunn family spokesman added: “Frankly, we have learned to take whatever the Trump administration says with a pinch of salt.

“There are extradition proceedings under way, and whether the British authorities’ decision yesterday was helpful or not to the American government, this case will be dealt under the rule of law, which we know is just as important to Americans as it is to British people.

“To the extent that anyone in authority seeks to impede a lawful request from the British for Anne Sacoolas to be brought back to the UK, they will ultimately have to put their argument to an independent judge in court.”

Meanwhile, asked on Saturday during a visit to Estonia if Sacoolas should be extradited to the UK, Boris 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.”