An expert on diplomacy has described the diplomatic immunity claim from Harry Dunn’s alleged killer as a “palpable absurdity”.

Anne Sacoolas claimed diplomatic immunity after the teenager died in a crash outside a US military base in Northamptonshire on August 27.

Sir Ivor Roberts, a former British ambassador in Serbia, Ireland and Italy, said it was “wholly illogical … to waive immunity for a member of staff” at RAF Croughton and “not to do so for a member of his or her household”.

Sacoolas’s claim to immunity is set to be the subject of a judicial review trial in which Mr Dunn’s parents, Charlotte Charles and Tim Dunn, allege Foreign Secretary Dominic Raab acted unlawfully over the suspect’s departure from the UK.

The Dunn family claims that Sacoolas – in line with agreements made between the UK and the US – should have had her claim to immunity pre-waived.

The Foreign Office argues that – because Sacoolas was notified to them as a spouse – and dependants were not mentioned in the so-called Exchange of Notes, her immunity was not pre-waived, in what they have described as an “anomaly”.

A former junior minister at the Foreign Office (FCO), Sir Tony Baldry, who signed the agreement with the US covering RAF Croughton, also said he believed FCO lawyers would not have “created a situation whereby immunity was waived for agents outside work, but not for their spouses”.

In his witness statement, he 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.”

Sir Tony’s evidence will be heard at a judicial review trial in October or November but Lord Justice Flaux dismissed an application to include Sir Ivor’s evidence.

Before the application was dismissed, Sir Ivor said in a statement: “It is wholly illogical and against any sensible interpretation of the Vienna Convention to waive immunity for a member of staff and not to do so for a member of his or her household.

“This could lead to the absurd position whereby if Mr Sacoolas had taken a Sunday afternoon drive, ie. not on official business, and had had the accident with Harry Dunn, he would not enjoy immunity from criminal jurisdiction but were his wife driving and he were the passenger, she would enjoy that immunity.

“In a more extreme example of domestic violence, were the US official to kill his wife after a domestic argument he would not enjoy immunity from criminal jurisdiction whereas if the roles were reversed and she killed him, she would benefit from immunity. A palpable absurdity.”

Speaking on behalf of the Dunn family, spokesman Radd Seiger told the PA news agency: “It is now overwhelmingly clear how Harry Dunn’s parents find themselves in the appalling situation they are in, without justice, without closure.

“They lost their son 10 months ago and the person who took his life was allowed to walk away.

“But the minister responsible at the time and one of this country’s leading diplomatic immunity experts now make it clear that Anne Sacoolas did not have diplomatic immunity at the time of the crash with Harry Dunn.”

A spokeswoman for the Foreign Office said: “Anne Sacoolas held diplomatic immunity on arrival in the UK on July 24 until her departure from the UK.

“Under the Vienna Convention on Diplomatic Relations any waiver of immunity must be express. The historic arrangements covering Croughton contained no such waiver for family members.”

Responding to Sir Tony’s comments, the spokeswoman continued: “Tony Baldry himself admits he had no recollection of the submission and does not recall seeing the final agreement.

“There is no pre-waiver of immunity for spouses in the Croughton agreement. The FCO identified this anomaly, after the tragic case of Harry Dunn.

“The Foreign Secretary instigated a full review of those historic arrangements – presided over by previous governments – and is now working to correct the anomaly.”