The High Court has refused an application by Harry Dunn’s parents for the Foreign Office to disclose evidence relating to a “secret agreement” between the UK and US governments which is said to have given diplomatic immunity to their son’s alleged killer.

Charlotte Charles and Tim Dunn are bringing legal action against Foreign Secretary Dominic Raab and Northamptonshire Police, claiming they acted unlawfully over the departure of Anne Sacoolas from the UK.

Mr Dunn, 19, was killed in August last year when his motorbike crashed into a Volvo driving on the wrong side of the road outside RAF Croughton in Northamptonshire.

Sacoolas, whose husband Jonathan Sacoolas worked as a technical assistant at the base, claimed diplomatic immunity following the crash and was able to return to the US.

The 42-year-old was charged with causing death by dangerous driving in December but an extradition request was refused by the US the following month.

Mr Dunn’s parents argue that the Foreign Office “acted unlawfully by proceeding as if Anne Sacoolas conclusively had immunity and/or advising other state bodies that she did”, which they say prevented Northamptonshire Police “from reaching an informed view as to the immunity issue”.

Following a preliminary hearing on Thursday, conducted remotely by video call, Lord Justice Flaux and Mr Justice Saini rejected the application for the Foreign Office to disclose further evidence ahead of a full hearing.

Lord Justice Flaux said: “We do not consider that any of the documents sought is necessary for the fair and just determination of the issues in the case, so that application is refused.”

The court also refused an application to rely on expert evidence from Sir Ivor Roberts, “one of the country’s most distinguished diplomats”.

Lord Justice Flaux said: “Whilst we, of course, acknowledge his eminence, we do not consider any of that evidence to be admissible or relevant to the issues we have to decide.”

Asked if the refusal of the application for further disclosure documents affected the family’s case, their spokesman Radd Seiger told the PA news agency: “It doesn’t affect the parents’ case against either the police or Dominic Raab at all.

“The reality is, and I think anybody who observed that hearing today saw that decisions were made at the Foreign Office shortly after Harry’s death that should not have been made.

“The other development of course is that the parents have now been given permission to bring a case against Dominic Raab alleging that he obstructed the police’s lawful investigation and that’s a very serious matter.”

The court directed that a full hearing of Mrs Charles and Mr Dunn’s case will be heard in October or November.

Geoffrey Robertson QC, representing Mrs Charles and Mr Dunn, earlier told the court that the Foreign Office “has through its actions obstructed a criminal investigation, under pressure from the United States”.

He argued that Mr Sacoolas had “only limited” immunity from prosecution in relation to “acts performed in the course of his duties”, and that his wife entered the UK with “none at all since she would perform no duties relating to the mission”.

Mr Robertson said the case “turns on the interpretation of a secret agreement made in 1995, and updated in 2001, between the US and UK as a result of a US request to add up to 200 technical officers as diplomatic agents at RAF Croughton”.

He said that, in 1995, the UK was “deeply concerned about this unprecedented request and the danger of media interest if crimes (in particular, road traffic-related crimes) were committed by agents and/or their dependants”.

Reacting to a 1995 briefing note which “prophesied” her son’s death, Mrs Charles told PA: “We were beyond angry when we read that – absolutely beyond angry.

“Twenty-five years on, nobody has done anything. The very scenario that they were worried about has happened.

“That just makes me feel that us regular UK citizens, that work hard and pay our taxes, are just left at the bottom of the pile when the Government is meant to be there to look after you.

“It’s despicable that those concerns were raised all those years ago and nothing has ever been done.

“But it’s time now – something has to change. It has to change.”

Asked for her thoughts on Thursday’s High Court hearing, Mrs Charles told PA: “Hopefully we should have our court case during October or November at the latest and I would just say as a mum, having lost Harry, I would plead with the UK Government, Anne Sacoolas and President Trump to work together on this to bring her back.

“It’s extremely clear to us that we have a strong case going forward and I’m happy.”

Mr Robertson asked the court to order the Foreign Office to disclose historical documents about the 1995 agreement, as well as notes of all phone calls and other messages about Mr Dunn’s death involving Foreign Office officials.

He submitted: “It is important that the claimants and the court are able fully to understand why the Foreign Office appears to have concealed information from Northamptonshire Police and conceded the position in relation to diplomatic immunity contended for by the Americans.”

In his opening remarks to the court, Mr Robertson said there had been a number of “inaccurate reports” in the media that the court would be asked to determine the diplomatic immunity issue at the hearing on Thursday.

He added: “Perhaps I will be allowed to say that it will not, it will be looking only at the question of whether the defence should disclose certain documents.”

Sir James Eadie QC, for the Foreign Office, said Mr Raab “has acted lawfully at all times”.

He argued that the Foreign Office “has already given extensive disclosure, well in excess of what is strictly required” and the claimants’ request for further material should be refused.

Sir James also rejected the “serious allegation” that the Government had failed to disclose relevant documents to Mr Dunn’s parents, saying: “There is no basis whatever for the repeated assertion, both in these proceedings and the media, that the Secretary of State has failed in any way to comply with his duty of candour.”

Reflecting on the campaign so far after their first court hearing on Thursday, Harry’s father told PA: “To get this far, this quickly, considering the people we are up against, is absolutely fantastic and we can’t thank people enough really.

“I feel like we’re getting closer to what happened and why it happened.

“We’re going to push on and everybody is going to know the truth about what happened to Harry.”