A Foreign Office official raised concerns over the potential for a car crash involving a diplomatic immunity claim 25 years before the death of Harry Dunn, documents before the High Court have revealed.

Government fears over accidents outside RAF Croughton came to light as the teenage motorcyclist’s family had their first court battle with the Foreign Secretary Dominic Raab and Northamptonshire Police on Thursday.

Charlotte Charles and Tim Dunn allege that suspect Anne Sacoolas – in line with agreements made between the UK and the US – should have had her claim to immunity pre-waived.

In a so-called “Exchange of Notes” between the two countries in 1994-95, an agreement was made over immunity for administrative and technical staff at RAF Croughton “on the understanding that the United States government … waives the immunity … in respect of acts performed outside the course of their duties”.

But another document disclosed by the Foreign Office (FCO) – a briefing note sent to Sir Tony Baldry, then a junior FCO minister, in May 1995 – concerns were raised that “an accident involving the claim of immunity could make the local if not national headlines”.

Sir Tony said in a witness statement for the High Court case that he believed FCO lawyers at the time would not have “created a situation whereby immunity was waived for agents outside work, but not for their spouses”.

In his statement about the immunity granted to staff at RAF Croughton, Sir Tony said: “We were obviously extremely unhappy at the prospect of technicians and their dependants being placed above the law, and this I made clear by instructing that any agreement must be conditional upon the waiver.

“I am sure that the US did not and would not have raised any specific request for dependants to be exempted from the law – had they done so I would have refused or at the very least referred this matter to the Secretary of State for him to decide.

“I cannot imagine any government agreeing to such an arrangement.”

The Dunn family recruited the former Tory MP for their case against Mr Raab – in which they allege that the Foreign Secretary acted unlawfully over the departure of suspect Sacoolas.

The 42-year-old American was charged with causing Harry’s death by dangerous driving in December last year following the crash outside RAF Croughton in August.

She claimed diplomatic immunity and was able to return to the US on September 15, 19 days after the teenager’s death.

Documents also showed that a representative of the Crown Prosecution Service expressed their doubts to the FCO and police that the UK had the “political leverage” to persuade the US into a “change of heart”.

The September 20 email read: “Sadly, I have to agree with your view that it is highly unlikely that any prosecution can be brought in this matter.

“If it is the position that the suspect has diplomatic immunity and that the US is declining to waive the same, that would amount to a complete bar to prosecution.

“It is difficult to see what further steps could be taken. I do not know whether there are lines of ‘appeal’ open to the FCO.

“It seems to me that the only possibility would be for the US to have a change of heart and I doubt whether the UK has the political leverage to make that happen.”

Court documents – previously seen by the PA news agency before the hearing – showed that a senior diplomat at the FCO had sent a text message to their US Embassy counterpart saying they should “feel able” to put Sacoolas on the next flight home.

Initial disclosure documents, which surfaced earlier this year, showed a briefing note copied to Mr Raab’s private secretary from three days after the fatal crash – which revealed concern for some “very unpalatable headlines”.

A witness statement from the senior investigating officer at Northamptonshire Police said an official at the FCO had requested the force delay telling the family that a waiver for Sacoolas’s diplomatic immunity had been declined by the US – adding that it would help if they could get their “ducks in a row” beforehand.

At a preliminary hearing on Thursday, conducted remotely via Skype, the family are asking Lord Justice Flaux and Mr Justice Saini to order the Foreign Office to disclose further evidence ahead of a full hearing.