Britain and the US agreed to remove an “anomaly” of diplomatic immunity that has so far allowed an American citizen to avoid prosecution in the UK over the death of a teenager in a car crash.
Anne Sacoolas, the wife of an American diplomat, was involved in an accident that killed Harry Dunn, 19, but the US State Department said she was immune from prosecution.
Police believe Ms Sacoolas was driving on the wrong side of the road as she left RAF Croughton, a military base in Northamptonshire, England, and collided with the teenager's motorcycle.
UK Foreign Minister Dominic Raab said that under new arrangements American staff working at the military base could face prosecution.
Dunn's mother, Charlotte Charles, said the change was a "huge step forward" and that the family's campaign for Ms Sacoolas to face criminal proceedings in the UK would continue.
Ms Sacoolas left the country shortly after the crash and despite later being charged with dangerous driving there has been no indication she is willing to return for a court case.
British authorities were frustrated that diplomatic immunity was invoked in the case and the Dunn family were angry Ms Sacoolas fled before being charged in connection with the crash.
"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 heartbreaking case of Harry Dunn," Mr Raab said.
The new rules began on July 20 but it is understood the change will not be backdated so it can apply to Ms Sacoolas.
But human rights lawyer Adam Wagner said it was still an important change.
In a written statement, Mr Raab said: “The US waiver of immunity from criminal jurisdiction is now expressly extended to the family members of US staff at Croughton, thus ending the anomaly in the previous arrangement and permitting the criminal prosecution of the family members.
“The waiver now extends to all embassy staff serving at Croughton in respect of action outside their official duties.”