Harry Dunn's parents have lost their High Court battle against the Foreign Office over whether their son's alleged killer had diplomatic immunity.
Mr Dunn, 19, died when his motorbike was in a crash with a car near RAF Croughton, Northamptonshire, in 2019.
The suspect, 43-year-old Anne Sacoolas, later left for the United States citing diplomatic immunity.
Mr Dunn's mother Charlotte Charles said the High Court ruling was "just a blip along the way".
Mrs Sacoolas, whose husband Jonathan worked as a technical assistant at the RAF base, was charged with causing death by dangerous driving in December, but an extradition request was denied in January.
The legal challenge by Mrs Charles and Mr Dunn's father Tim Dunn claimed Mrs Sacoolas should not have been granted immunity.
But Lord Justice Flaux and Mr Justice Saini concluded "that Mrs Sacoolas enjoyed immunity from UK criminal jurisdiction at the time of Harry's death".
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."
Family spokesman Radd Seiger said Mr Dunn and Mrs Charles would be appealing against the ruling.
Foreign Secretary Dominic Raab said: "While this judgment makes clear the Foreign Office acted properly and lawfully throughout, I appreciate that won't provide any solace to the family in their search for justice.
"We stand with them, we're clear that Anne Sacoolas needs to face justice in the UK, and we will support the family with their legal claim in the US."
The High Court judges also rejected a claim by Mr Dunn's parents that the Foreign Office had "usurped" Northamptonshire Police's investigation into their son's death.
Lord Justice Flaux and Mr Justice Saini found officials had "sought to assist rather than obstruct Northamptonshire Police in their investigation".
The family's case centred on a 1995 agreement between the UK and the US, granting immunity to administrative and technical staff at the RAF base, which the US waived in relation to "acts performed outside the course of their duties".
Lawyers for Mr Dunn and Mrs Charles argued that the Foreign Office "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".
In written submissions, the Foreign Office argued its officials had "objected in strong terms" to Mrs Sacoolas leaving the UK, and "repeatedly emphasised" that the department "wanted the Sacoolas family to co-operate with the UK authorities".
'Cruel and needless'
Lord Justice Flaux and Mr Justice Saini ruled that Mrs Sacoolas had diplomatic immunity "on arrival in the UK" under the Vienna Convention on Diplomatic Relations.
As this had not been "expressly waived", it meant Mrs Sacoolas "had immunity at the time of Harry's death" on 27 August, they said.
Mr Dunn said: "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."