The foreign secretary, Dominic Raab, has met the American ambassador and urged the US to “reconsider its position” on the diplomatic immunity given to a suspect in the case of a teenager killed in a road crash.

He spoke to Woody Johnson on Tuesday to urge him to “do the right thing” by the family of 19-year-old Harry Dunn, who died after a car crashed into his motorbike on 27 August.

Despite Raab’s efforts, the Foreign Office appears increasingly pessimistic that Anne Sacoolas, the wife of a US intelligence officer, will return to the UK to answer questions about her role in the crash, even though some legal experts have challenged the US claim that she is entitled to full diplomatic immunity.

US state department officials, in private and public exchanges, have expressed regret about Dunn’s death outside an RAF base used by the US air force but said it was global American policy not to issue immunity waivers in such cases.

Before she left the UK, Anne Sacoolas had been helping British police with their inquiries into the 27 August collision between Dunn’s motorcycle and her Volvo XC90 near RAF Croughton, in Northamptonshire. They believed she had driven on the wrong side of the road for 400 metres after leaving the base.

Anne Sacoolas.

The Foreign Office insisted her husband, Jonathan Sacoolas, had full diplomatic immunity under the Vienna convention, even though his name was not included in the US diplomatic list, according to an expert in diplomatic law.

UK diplomats said that list was a far from exhaustive register of those capable of claiming full diplomatic immunity, and the RAF Croughton base where Jonathan Sacoolas had worked was treated as an annex of the US embassy.

Boris Johnson has said Sacoolas should face justice in the UK, saying: “I do not think that it can be right to use the process of diplomatic immunity for this type of purpose.”

The prime minister has promised to raise the issue with Donald Trump if Raab, was unable to make progress in persuading US officials to make Sacoolas return to the UK to face police. Raab raised the case with the US secretary of state, Mike Pompeo, on Monday night.

Q&A How does diplomatic immunity work? Show Hide Diplomatic immunity is the protection given under international and UK law to foreign diplomats and their families. It was formalised through the 1961 Vienna convention on diplomatic relations (VCDR). Reciprocal global agreements also protect UK diplomats working abroad. Without a foreign state agreeing to lift immunity, diplomats and their dependent families can only be detained “as a last resort” if, for example, they are deemed to be a danger to others or themselves. The only other sanction available to the Foreign Office would be to order their expulsion. When a foreign state agrees to remove that legal protection, it is known as a “waiver of immunity”. Individual diplomats cannot do so; the embassy of the foreign state has to make a formal request. As well as covering diplomats and their families anywhere in the UK, the same convention and legislation prevents UK officials from entering diplomatic premises. That inviolability enabled Julian Assange to stay in the Ecuadorian embassy in London for seven years. Owen Bowcott, Legal affairs correspondent

But Mark Stephens, an acknowledged legal expert on diplomatic immunity law, challenged whether Jonathan Sacoolas enjoyed full immunity under the Vienna convention, saying he had not been registered as such with the Foreign Office.

He suggested Sacoolas and his dependents instead only had more limited immunity under the Visiting Forces Act, and its derivative bilateral orders that provide immunity to personnel on specific UK bases. That immunity, according to Crown Prosecution Service guidance, is restricted to specific offences that “arose out of and in the course of the service person’s duties as a member of the visiting force including whether the act occurred in the course of the individual’s professional duties”.

The US air force has in the past, according to the CPS, interpreted professional duties to include driving from an air base to a home address. The Sacoolas family was reportedly living on the base, which would mean they could not claim that defence.

Offering to speak to the grieving family, Stephens said: “The bilateral treaty doesn’t bind Northamptonshire police and it does not bind the family of this poor boy who was deceased, and as a consequence of that they can apply for her extradition to come back to face manslaughter charges.”

He said the family had a right to sue Anne Sacoolas outside the UK jurisdiction and suggested the US had tried to get the intelligence officer out of the country to keep his identity secret, a tactic that had backfired: “If she had gone to court in the UK, probably no one would have known about him.”

Charlotte Charles, Dunn’s mother, insisted in interviews broadcast on Tuesday that the family would not cease its efforts to seek justice for her son, including changes to the way in which diplomatic immunity was interpreted in the UK. She said they were raising funds to take their case to the US.

Play Video 0:58 Harry Dunn's parents call for diplomat's wife to face justice over son's death – video

Charles said: “We are hoping that with Boris [Johnson] now clearly on his side and saying that he also feels it is not right, we are very much hoping that over the next couple of days things will improve for us and it will help our fight to get some justice done.

“And also we are hoping that it will help change this for the future. We are doing this because Harry would want to fight for what he believed in. And we need to fight for change.

“It is going to take quite a long time to come to terms. The part that I struggle to understand [is] why she took the decision to leave, why she felt it was the right thing to go.”

Charles said that understanding would only come when “we have a chance to meet her and for her to talk about it, see her remorse”.

The motorcyclist’s father, Tim Dunn, said: “She knew Harry had died, she had been told that, and what I really struggle to understand is if you knew that you have killed somebody, you think it is the right thing to flee the country.”

The US state department’s own guidelines for law enforcement officials make it clear that the US does not believe diplomatic immunity extends to foreign diplomats committing serious or repeat driving offences in the US, either while under the influence of alcohol or not.

Sacoolas, who reportedly had her 12-year-old son in the car when the crash happened, was spoken to by police the day after the crash and initially said she had no plans to leave the UK.

Police officers became aware immunity was an issue because her husband was a diplomat and applied to the US embassy for a waiver to allow them to question her. But they were later informed the waiver had been refused and Sacoolas had left the country.