Britain and America have agreed to end the “anomaly” that allowed an alleged death crash driver to claim diplomatic immunity.
Motorcyclist Harry Dunn, 19, was killed when his bike was in collision with a car outside the Croughton US military base in Northants.
The driver was Anne Sacoolas, the wife of an American intelligence officer.
Ms Sacoolas, 43, fled Britain and has refused to return to help police with their inquiries.
Foreign Secretary Dominic Raab said changes had now been made to prevent that happening to anyone in a comparable situation.
However, despite the “anomaly” being amended, Ms Sacoolas remains in the US.
Mr Raab said in a written statement:
“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.”
Harry’s mother Charlotte Charles said the decision was a “huge step forward”.
She added that one of her campaign aims was for this to “never happen to another family again”.