Harry Dunn's parents have formally filed for a judicial review into the Government's handling of the diplomatic immunity row surrounding the death of their son.
The legal suit comes as the Northamptonshire family hopes to meet Prime Minister Boris Johnson to discuss the Foreign and Commonwealth Office's (FCO) conduct since the crash with Anne Sacoolas outside Croughton in August.
Foreign Secretary Dominic Raab has reasserted to them that the Government will seek full costs of the legal proceedings, which could be more than £50,000, according to the family.
Family spokesman Radd Seiger wrote on the Justice4Harry Facebook page: “The parents have done everything physically within their power to avoid having to sue the FCO.
"There have been repeated public and private attempts on our part to engage with those in authority to resolve this dispute amicably.
"When we did engage with Mr Raab, he lied and misled to the parents at a meeting on October 9, telling them that Anne Sacoolas had diplomatic immunity, when the very day before on October 8, he had agreed with the US Government that immunity no longer applied.
"He even told mum Charlotte that he would put in a third request for a waiver.
"The parents believe he should no longer hold any public office, let alone high public office."
Mr Johnson told ITV News Anglia he could not comment on the FCO's conduct but insisted the Government is doing all it can for the family and to get Mrs Sacoolas to face justice.
Mr Seiger has called for the prime minister to meet Harry's parents to answer their questions, which is apparently being facilitated by now-prospective South Northamptonshire MP Andrea Leadsom.
Harry's parents believe Mr Raab was wrong to accept the US’ interpretation of the law around diplomatic immunity, informing their advice to Northamptonshire Police that Mrs Sacoolas had immunity.
"It is for the courts, not the Foreign Secretary, to decide whether someone has diplomatic immunity," the Facebook post says.
"It is important that the High Court rule conclusively that she did not. This will help in the family’s battle to seek Mrs Sacoolas’ extradition and get justice for Harry."
The family also wants the judicial review to clarify the laws on diplomatic immunity laws for other US intelligence staff and their families, including those at RAF Croughton.
They also claim the treaty which applies to the Northamptonshire airbase breaches the UK’s human rights obligations, particularly the right to a fair trial and the right to life.
A Foreign Office spokesperson said: “We have deep sympathy for Harry’s family. We have done and will continue to do everything we properly can to ensure that justice is done.
“As the Foreign Secretary set out in Parliament, the individual involved had diplomatic immunity whilst in the country under the Vienna Convention on Diplomatic Relations.”
Mr Raab was heckled after Harry's family was reportedly not allowed into a hustings at a church in Surrey on Monday evening - the church said it was due to overcrowding concerns.