Harry Dunn’s parents say they are appealing a High Court ruling that their son's alleged killer had diplomatic immunity, after taking "strong legal advice".
Judges accepted the Foreign Office's position that American suspect Anne Sacoolas "enjoyed immunity from UK criminal jurisdiction" as part of their judgment, which was handed down on Tuesday.
Mr Dunn, 19, died after his motorbike was involved in a collision outside RAF Croughton in Northamptonshire last August.
The Royal Air Force base is home to a US Air Force communications station.
Anne Sacoolas, whose husband, Jonathan Sacoolas, worked as a technical assistant at the base, left the country a few weeks later after the United States said she was entitled to diplomatic immunity.
Ms Sacoolas was ultimately charged last December with causing death by dangerous driving, but an extradition request was rejected by the US State Department in January – a decision it later described as "final".
Her lawyers later admitted she had driven on the "wrong side of the road for 20 seconds" before the incident.
Mr Dunn’s parents, Charlotte Charles and Tim Dunn, claimed the Foreign, Commonwealth and Development Office (FCDO) wrongly decided that Sacoolas had diplomatic immunity and unlawfully obstructed Northamptonshire Police’s investigation into their son’s death by keeping the force "in the dark".
In a High Court judgment delivered on Tuesday, Lord Justice Flaux and Mr Justice Saini said: "Our conclusion is that Mrs Sacoolas enjoyed immunity from UK criminal jurisdiction at the time of Harry’s death."
The judges also rejected Mrs Charles and Mr Dunn’s claim that the FCDO "usurped" Northamptonshire Police’s investigation into their son’s death, finding officials "sought to assist rather than obstruct Northamptonshire Police in their investigation".
Mrs Charles and Mr Dunn’s case centred on a 1995 agreement between the UK and the US, granting immunity to administrative and technical staff at RAF Croughton, which the US waived in relation to "acts performed outside the course of their duties".
At a hearing earlier this month, their lawyers said the FCDO "took upon itself the authority to resolve the question of immunity and ultimately and unlawfully decided to accept the US embassy’s decision that Anne Sacoolas had immunity".
Sam Wordsworth QC told the court that Sacoolas had “no duties at all” at the base and therefore “never had any relevant immunity for the US to waive”.
Lord Justice Flaux and Mr Justice Saini found that Sacoolas had diplomatic immunity "on arrival in the UK" under the Vienna Convention on Diplomatic Relations (VCDR) which had not been "expressly waived", meaning she "had immunity at the time of Harry’s death".
The judges said: "We do not come [to] this conclusion with any enthusiasm for the result, but it is compelled by the operation of the VCDR."
Speaking after the ruling, Mrs Charles said: "The Government and Mrs Sacoolas need to understand that this court ruling is just a blip along the way.
"I promised my boy I would get him justice and that is just what we are going to do. No-one is going to stand in our way."
She added: "It’s obviously disappointing that this court did not find in our favour but we are more focused now than ever on fulfilling our promise."
Harry Dunn's father said: "I still wake up every morning in absolute disbelief that we are in this situation at all.
"It’s bad enough feeling the horrible pain of not having Harry around and missing him, but I can’t believe the governments are putting us through this.
"It all seems so cruel and needless and I am just as angry today as I ever have been, but so determined to see it all through until we have justice."
In October, Harry Dunn's mother urged then-presidential candidate Joe Biden to reconsider America’s position on the woman who allegedly caused the teenager's death if he won the US election.
The same month, UK Transport Secretary Grant Shapps commissioned a review into road safety around US Visiting Forces bases in the UK as a result of Harry’s death.
A US Visiting Forces base is one in the UK which is used by the American military but remains the property of the British Government.