The foreign secretary, Dominic Raab, has announced the closing of secret legal loophole that allowed Harry Dunn’s alleged killer Anne Sacoolas to claim diplomatic immunity. A lawyer for Dunn’s family welcomed the move as a “huge step” in securing the extradition to the UK of Sacoolas to stand trial. Sacoolas, the wife of a CIA agent who was working at the US intelligence base RAF Croughton in Northamptonshire, was allowed to return to the US days after allegedly driving her car on the wrong side of the road on 27 August and crashing into Dunn on his motorcycle. In a written statement to parliament, Raab said he and US and UK officials had agreed new arrangements to prevent family members of staff stationed at the base from claiming immunity from prosecution in future. His statement said: “We have secured the agreement of the US so that the Croughton arrangements could not in future be used in the same away as in the tragic case of Harry Dunn. These changes took effect by way of an exchange of notes on 20 July.” He said the US base had agreed to provide mandatory driving training to staff and to improve road signage to remind them to drive on the left. Raab said: “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 bring some small measure of comfort.” On TuesdayRaab had prompted anger from Dunn’s family by greeting the US secretary of state, Mike Pompeo, in Downing Street. Speaking at the gates of Downing Street on Tuesday, Harry’s mother, Charlotte Charles, said: “Words are not enough any more.” The family has mounted a judicial review of the Foreign Office’s decision-making in the case. Charles and her lawyer Radd Seiger met Raab on Wednesday to discuss diplomatic immunity at the base. Speaking to the Guardian afterwards, Seiger welcomed Raab’s statement as a “very significant development”. He said: “Huge credit to Harry’s parents for standing up, bringing this campaign and a judicial review, because without those none of these secret agreements at Croughton would have been brought to light. And it is absolutely unprecedented from an international relations perspective to get these pre waivers, but as I understand it anybody who commits any serious crime coming off Crouton now will be arrested, and we have the parents to thank for that.” He added: “Now that they’ve reached this agreement, how can the Americans with any credibility refuse now to send Anne Sacoolas back to the UK? The arrangements that apply in July 2020 are equally as good those in August 2019 when Harry Dunn died. This is a huge step forward towards bringing Anne Sacoolas back.” In the meeting, Raab told Seiger and and the Dunn family that the US was still resisting extraditing Sacoolas to the UK. Seiger said: “I am now going to work with him [Raab] to make sure that he brings further pressure to bear in terms of suspending the extradition treaty with the United States or there have to be consequences.” Raab’s statement confirmed that legal “arrangements” at the Croughton base “contained a waiver of immunity from criminal jurisdiction for US staff outside the course of their duties, but no such waiver for their family members.” He added: “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.” Raab also said the Department of Transport had launched a safety review of roads around the 10 US air force bases in England.