Family also refer Northamptonshire police to watchdog over investigation of sons death
The family of Harry Dunn will shortly issue a letter of claim to begin legal action against the Foreign Office, their spokesman has said.
They have also referred Northamptonshire police to the Independent Office for Police Conduct (IOPC) over the investigation into the death of their son, Radd Seiger said.
Dunn, 19, died after a collision outside RAF Croughton involving Anne Sacoolas, the wife of a US intelligence official who claimed diplomatic immunity and returned to her home country.
After the Dunn family held a meeting with the human rights barrister Geoffrey Robertson QC on Thursday, Seiger told Sky News: It was a very detailed, thorough and lengthy meeting The first action we will be taking is against the Foreign and Commonwealth Office.
The family would issue the letter as a prelude to seeking a judicial review.
Seiger said: We are absolutely clear that the Foreign Offices decision to advise Northamptonshire police that Mrs Sacoolas had the benefit of diplomatic immunity was unlawful and we will be seeking a judicial review of that decision to have it quashed.
Seiger said: Its absolutely clear that Mrs Sacoolas was involved in a very serious collision that ended in the loss of Harrys life.
We are clear shes admitted her culpability, that she committed a very serious crime that night. At the very least causing death by careless driving and at the worst causing death by dangerous driving. She is a fugitive from this country and she is on the run.
Seiger said the family would continue to appeal for Sacoolas to return to the UK.
The Dunn familys spokesman also questioned if Sacoolas was entitled to diplomatic immunity through her husband, saying: This person, whoever he is, was not a diplomat and therefore does not have the benefit of diplomatic immunity and nor does his family.
Seiger said the family planned to refer Northamptonshire police to the IOPC.
He said that although the force have not disclosed all the information this family are entitled to, we have deep concerns about the manner in which this investigation was conducted, and simply adds insult to injury to this family at their darkest hour.
A Foreign Office spokesperson said: We have done everything we can properly to clear a path so that justice can be done for Harrys family. As the foreign secretary set out in parliament, the individual involved had diplomatic immunity while in the country under the Vienna convention on diplomatic relations. We will respond to any legal action in due course.