A Downing Street staff member labelled the spokesman for Harry Dunn’s family a “bad guy” after he voiced concerns of a potential cover-up over the teenager’s death.
The internal email sent by an employee at number 10, seen by the PA news agency, followed a statement on Twitter from Radd Seiger in which he said the teenager’s family were intent on exposing “misconduct… on both sides of the Atlantic”.
Commenting on the email, a Downing Street spokeswoman said: “We apologise for the content of this email, which was inappropriate.”
Mr Dunn’s mother, Charlotte Charles, has demanded an explanation from the Prime Minister and a personal apology to Mr Seiger and her family.
Her 19-year-old son has been the subject of an international controversy after the US asserted diplomatic immunity for his alleged killer following a road crash outside RAF Croughton in Northamptonshire in August last year.
Anne Sacoolas travelled back to her home country on a commercial flight almost three weeks after the collision – with the family campaigning for her return to the UK ever since.
Lawyers acting on behalf of the 43-year-old said she had been driving on the wrong side of the road for around 20 seconds before the crash – saying she could not see Mr Dunn because of the “crest of a hill”.
She was charged with causing death by dangerous driving in December, but an extradition request submitted by the Home Office was rejected by the US State Department in January.
After the family and Mr Seiger met with President Trump in the Oval Office in October, the spokesman released a short statement on Twitter – in which he said: “The search for justice has now expanded beyond simply Mrs Sacoolas’ return, as important as that is.
“The family is now concerned that there has been misconduct and a cover up on both sides of the Atlantic and they are intent on exposing it with the help of their lawyers.”
The statement prompted an email from the unnamed Downing Street staff member – saying: “This guy is bad!”
PA understands the sender of the email was not directly involved with the case.
Reacting to the email, Mrs Charles told PA: “How dare they.
“It is beyond outrageous that our friend of 20 years, our neighbour, the one person who stood up to help was having his character and motivation called into question by someone at Number 10, who clearly did not have the courage him or herself to state publicly that what had happened to us was wrong – unlike Radd.
“I have demanded an urgent meeting with Boris Johnson and will expect a full explanation, an apology to both Radd and the family, and for heads to roll.
“It is completely and utterly unacceptable.”
Mr Seiger has represented the family since September last year and has not been paid for his part in the campaign.
Also commenting on the email, he told PA: “The comment from Number 10 followed a tweet I put out that the parents were concerned that there had been misconduct in allowing Anne Sacoolas to leave the UK and if so that we would expose it.
“What on Earth is someone in the Prime Minister’s office doing calling someone who was trying to help reverse an obvious injustice a bad guy?
“They must now come clean over what really happened and state publicly what they are going to do to put things right.
“Whoever wrote that email has no business occupying a place in this administration.”