The mother and twin brother of Harry Dunn have said they are "angry" after letters from the Prime Minister arrived two days before a court battle.
Charlotte Charles described the letters written to her and Niall Dunn as a "cut and paste response", adding that she remains "bitterly disappointed" Boris Johnson has not met with the family.
The family are set for a judicial review hearing with the Foreign Office (FCO) and Northamptonshire Police on Thursday in which they will dispute the diplomatic immunity claimed by suspect Anne Sacoolas.
Sacoolas, 42, was charged with causing Mr Dunn’s death by dangerous driving in December last year but an extradition request was refused by the US the following month.
Mrs Charles wrote to the PM on March 2 urging him "not to be frightened" of meeting with her and her family, saying there were "very serious problems" that needed addressing "for the benefit of the nation" after her son’s death outside RAF Croughton in Northamptonshire in August last year.
After receiving the letter on Tuesday, Mrs Charles said Boris Johnson "hasn’t even applied his own mind to what is going on and the reason why we asked for a meeting", before adding that "even President Trump saw it fit to meet us".
Mrs Charles said: "Having read both his letter to me and to Niall, I cannot begin to express how flat and angry I feel with this cut and paste response."
"This is the same nonsense that we have been hearing from other government departments for months."
She added: "Boris hasn’t even applied his own mind to what is going on and the reason why we asked for a meeting - even President Trump saw it fit to meet us."
"I can’t believe they have timed these letters to arrive on the eve of the Judicial Review just when we are trying to steady ourselves to deal with all that is coming."
In his letter to Mrs Charles, the PM said: "First of all, let me offer my deepest condolences for the tragic loss of your much-loved son, Harry.
"I cannot begin to imagine the pain that you and your family have gone through, and my heart goes out to you all.
"I know this loss was compounded by the decision of the United States not to waive immunity for Anne Sacoolas, then to refuse extradition.
"I feel strongly that this amounts to a denial of justice, and have said so publicly.
"I have raised Harry’s case personally with both President Trump and Secretary of State Pompeo, asking them to do the right thing. I will continue to do so."
"My son was killed by an American citizen, Mr Johnson and his Government let that person go and he doesn’t have the courage to meet with us.
Mr Johnson said the next steps of the legal process were "a matter for the Crown Prosecution Service".
In his letter to Harry’s twin brother Niall, Mr Johnson maintained the Foreign Office’s legal position, stating Sacoolas had diplomatic immunity and "neither the UK police nor the Government could lawfully prevent (her) from leaving the country".
In the letter to Mr Dunn, the PM said: "You ask to know the truth in this tragic case."
Mr Johnson continued: "Under arrangements put in place 25 years ago, and confirmed by another government six years later, there was an anomaly under which diplomatic immunity was waived in some circumstances for US staff at RAF Croughton but not for their families.
"As a result, Anne Sacoolas had diplomatic immunity and neither the UK police nor the Government could lawfully prevent Anne Sacoolas from leaving the country.
"We are negotiating those arrangements."
In response to the letter Niall Dunn said: "What is there to say?
"I asked him to grab a hold of the case because my parents were becoming increasingly upset at the Government’s handling of it.
"He has just ignored me and trampled all over my feelings.
"I’m so upset, really angry at him."
The Prime Minister told both Mrs Charles and Mr Dunn that the Foreign Secretary would be their lead point of contact but he continues to take a "close interest in the case".