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."
Dunn family fight for justice
Dunn family fight for justice
Undated family handout file photo of Harry Dunn. Harry Dunn's mother, Charlotte Charles, has marked the anniversary of her son's death by saying his name is "forever written into history" � and vowed to carry on his family's fight for justice.
Andrea Leadsom MP and Harry Dunn family spokesman Radd Seiger arrive to speak to the media in Brackley, Northamptonshire, where the MP called on Donald Trump to personally intervene to ensure suspect Anne Sacoolas faces trial over teenager Harry Dunn's death a year ago.
File photo dated 09/10/19 of Charlotte Charles, the mother of Harry Dunn, speaking to the media after leaving the Foreign and Commonwealth Office in London, where she met Foreign Secretary Dominic Raab. Harry Dunn's mother, Charlotte Charles, has marked the anniversary of her son's death by saying his name is "forever written into history" � and vowed to carry on his family's fight for justice.
Harry Dunn family spokesman Radd Seiger (left), Harry's mother Charlotte Charles (centre) and her partner Bruce Charles, speak with Andrea Leadsom MP (right) in Brackley, Northamptonshire, where the MP called on Donald Trump to personally intervene to ensure suspect Anne Sacoolas faces trial over teenager Harry Dunn's death a year ago.
File photo dated 20/12/19 of Harry Dunn's mother Charlotte Charles (centre) and stepfather Bruce Charles (left) outside the Ministry Of Justice in London after meeting with the Director of Public Prosecutions. Harry Dunn's mother, Charlotte Charles, has marked the anniversary of her son's death by saying his name is "forever written into history" � and vowed to carry on his family's fight for justice.
File photo dated 15/10/19 of the entrance to RAF Croughton, in Northamptonshire, near where Harry Dunn, 19, died when his motorbike was involved in a head-on collision in August last year. One year on from his death, the family of Harry Dunn continue to fight for justice in a case which has sparked diplomatic tensions between Washington and London.
File photo dated 23/04/20 of US President Donald Trump, who was told to "reconsider its position" on the immunity given to 42-year-old suspect Anne Sacoolas, following a road crash outside a US military base in Northamptonshire which resulted in teenage motorcyclist Harry Dunn's death. One year on from his death, the family of Harry Dunn continue to fight for justice in a case which has sparked diplomatic tensions between Washington and London.
Tim Dunn (far right) and his wife Tracey Dunn (second right), as they gather with friends and family at Portland Bill to scatter his son's ashes in his favourite place in Weymouth, Dorset, a month before the anniversary of his death.
File photo dated 20/12/19 of the family of Harry Dunn (left to right) mother Charlotte Charles, stepfather Bruce Charles, family spokesman Radd Seiger and father Tim Dunn. Harry Dunn's parents have dropped their legal claim against Northamptonshire Police after concluding the force are "absolved of any blame" following their son's death.
Harry Dunn's mother Charlotte Charles arrives at the Foreign and Commonwealth Office in Westminster, London, for a meeting with Foreign Secretary Dominic Raab.
Cardboard cutout of Northampton Town fan Harry Dunn who died in a car crash last August is seen in the stands during the Sky Bet League Two play-off semi final first leg match at the PTS Academy Stadium, Northampton.
Business Secretary Andrea Leadsom and Harry Dunn's mother Charlotte Charles outside South Northamptonshire Council offices in Towcester, after they met following the news that the US have refused to extradite Anne Sacoolas who is charged with causing his death in a road collision outside RAF Croughton in August last year.
A motorbike convoy makes it's way through the village of Charlton after following Harry Dunn's last ride as a tribute to the teenager who died when his motorbike was involved in a head-on collision near RAF Croughton, in Northamptonshire in August.
Charlotte Charles (left), the mother of Harry Dunn, Bruce Charles, and Tracey Dunn speak to media after a motorbike convoy followed Harry Dunn's last ride through Brackley as a tribute to the teenager who died when his motorbike was involved in a head-on collision near RAF Croughton, Northamptonshire, in August.
Chief Constable of Northamptonshire Police, Nick Adderley (centre) and Superintendent Sarah Johnson speaking during a press conference at Northamptonshire Police HQ at Wootton Hall Park, Northampton about the death of Harry Dunn.
Harry Dunn's Auntie Katie Grant (right) and Nicola Watson tie a Green Ribbons which are being made be members of the village of Charlton, Oxfordshire in memory of 19-year-old Harry Dunn who died while out riding his Green motorcycle near RAF Croughton.
Please Drive on Left signs and arrows have been placed on the B4031 road outside RAF Croughton, in Northamptonshire, where Harry Dunn, 19, died when his motorbike was involved in a head-on collision in August.
(Left to right) Bruce Charles and Charlotte Charles (Harry's mother), lawyer Radd Seiger, Tim Dunn (Harry's father) and Tracey Dunn at a press conference at the Parker New York Hotel in New York, US, where Charlotte Charles said that Anne Sacoolas, the American woman suspected of causing her son's death, should be brought back to the UK to face justice.
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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."