British Prime Minister Boris Johnson has said he will make a personal plea to the White House should a U.S. diplomats wife fail to return to the U.K. to face questioning in a wrong-way collision that killed a British teenager.

Johnson stood up for the family of 19-year-old motorcyclist Harry Dunn on Monday, telling reporters that Anne Sacoolas claim of diplomatic immunity is unjustified.

I do not think that it can be right to use the process of diplomatic immunity for this type of purpose and I hope that Anne Sacoolas will come back and will engage properly with the processes of laws that are carried out in this country, he told reporters . If we cant resolve it then, of course, I will be raising it myself personally with the White House.

Sacoolas, 42, reportedly fled back to the U.S. following the Aug. 27 traffic collision in the village of Croughton.