Britain has agreed with the United States to remove an "anomaly" which allowed the wife of a U.S. official to claim diplomatic immunity from criminal prosecution after she was involved in a road accident in which 19-year-old Briton Harry Dunn was killed.

The crash last August has caused friction between London and Washington after Britain criticized the United States for refusing to extradite Anne Sacoolas.

Now the loophole that allowed Sacoolas to claim immunity has been closed.

Britain's foreign minister Dominic Raab said that under new arrangements, the criminal prosecution of family members of American staff working at the Croughton Annex military base in central England would be permitted.

"It's important that we have now agreed with the U.S. new arrangements that have closed the anomaly that led to the denial of justice in the heart-breaking case of Harry Dunn," Raab said in a statement.

Dunn's family have said Sacoolas was driving on the wrong side of the road at the time of the crash, near the air force base in Northamptonshire which is used by the U.S. military.

Sacoolas left Britain shortly after the accident. Her lawyer has said that she will not return voluntarily to potentially face jail for "a terrible but unintentional accident."