The UK and US have agreed to “plug the loophole” that allowed Harry Dunn’s alleged killer to claim diplomatic immunity, the Foreign Secretary has said.

A court hearing last month heard that the “secret agreement” permitted Anne Sacoolas to return to her home country after a road crash which killed the 19-year-old in August last year.

Despite the “anomaly” now having been amended in the immunity agreements surrounding RAF Croughton, the Northamptonshire military base near where Harry Dunn died in a road collision, his alleged killer still remains in the US.

In a written statement issued on Wednesday, Dominic Raab said: “First and foremost, the US waiver of immunity from criminal jurisdiction is now expressly extended to the family members of US staff at the Croughton Annex, thus ending the anomaly in the previous arrangements and permitting the criminal prosecution of the family members of those staff, should these tragic circumstances ever arise again.

“We have the deepest sympathy for Harry Dunn’s family. No family should have to experience what they have gone through and I recognise that these changes will not bring Harry back.

“However, I hope that the knowledge that the Croughton arrangements have been revised and that a family in their position would now see justice done brings some small measure of comfort.”

Speaking to the PA news agency following the announcement, Harry’s mother Charlotte Charles said it was a “huge step forward” – adding that one of the family’s aims was for this to “never happen to another family again”.

She said: “We’re really pleased, really proud of what us parents have achieved.

“One of our campaign’s aims was to ensure that no other family can suffer the way we have.

“Harry most importantly would be very proud of us but we’ve still got a long way to go – we’ve still got to get Anne Sacoolas back.”

The Prime Minister’s official spokesman said there was “no change” to their position that Sacoolas should return to the UK, adding: “Justice must be done for Harry Dunn and his family and we continue to call for the return of Anne Sacoolas to the UK.”

The US State Department said the amendment of the diplomatic immunity arrangements at RAF Croughton “is a reflection of our especially close relationship” with the UK.

Mrs Charles said the US State Department’s words “mean nothing to her”.

She said: “I don’t understand what this special relationship is, when they’ve clearly just put their foot down on something that is just totally and utterly morally wrong.

“For them to agree to having this loophole plugged effectively, is surely admitting that they jumped through that loophole and abused that loophole almost 11 months ago.

“So their words don’t mean anything to me, I’m afraid.”

Giving her reaction to the announcement, shadow foreign secretary Lisa Nandy said it was “no substitute for complete transparency”.

She said: “The Foreign Secretary has still not come to Parliament to explain how failings in his department allowed a US citizen to leave the country while their immunity was still in question.

“Labour will continue to push for a full inquiry into the litany of failures in this case and demand the answers that Harry Dunn’s family deserve.

“This announcement is no substitute for complete transparency – if the Foreign Secretary is serious about ensuring no other family goes through such an ordeal, then he must be honest about the mistakes that were made in this case.”

Asked how it felt to have one of the campaign’s objectives completed, Mrs Charles told PA: “Emotional. Really emotional because we are undoubtedly tired and every day is still a struggle.

“I’ve been shaking ever since the news came out and I can’t explain why I feel so shaky.

“I’m thrilled that Dominic Raab and his team have managed to make sure that those pre-waivers are issued to anyone who commits the crime but at the same time I’m aware that we still have a huge task ahead of us.

“It’s definitely worth a mini-celebration and Dominic Raab and his team deserve some credit for that – but the only time I will ever give them full credit will be when they have worked with us to get Anne Sacoolas back.”

The teenage victim’s father, Tim Dunn, said he had “mixed emotions” after the announcement.

He told PA: “This is great news… but if it’s right for now, why was it not right 11 months ago?

“This has been changed because it was wrong – so if it was wrong, why can’t they admit it was wrong and send her back?

“It does grate a bit, because it doesn’t feel like it’s fair.

“Yes, it’s a good result. But they are not using the good result to get Anne back.”

In a statement released after the announcement, Northamptonshire Police said: “While we understand these changes will not be retrospective, Northamptonshire Police welcomes these changes.

“Northamptonshire Police remains committed to working with colleagues in the Crown Prosecution Service to ensure Anne Sacoolas is returned from the US to allow criminal proceedings to go ahead here in the UK.”

In a statement issued after the announcement, the Foreign Secretary said: “It’s important that we have now agreed with the US new arrangements that have closed the anomaly that led to the denial of justice in the heart-breaking case of Harry Dunn. The new arrangements mean it could not happen again.

“I know these changes won’t bring Harry back, and I appreciate the pain and suffering the family are still going through.

“But I hope this may bring some small measure of comfort to them, because I know they want to prevent any other family going through the same ordeal they have.”