Police have written to the United States embassy in London to demand immunity is waived for 42-year-old Anne Sacoolas, who left the United Kingdom after the crash despite telling officers she did not plan to do so.
CNN reported the diplomat's wife was allegedly driving on the wrong side of the road when the crash occurred. Dunn's family alleged that Sacoolas drove on the wrong side of the road for 400 yards before colliding with him.
On Monday (Tuesday New Zealand time), Johnson said that he hoped Sacoolas would return to Britain and "engage properly with the processes of law" while adding that he was willing to raise Dunn's case "personally with the White House", if necessary.
Speaking to Sky News UK, Harry's mother Charlotte Charles said the death of her son has left "a family in complete ruin".
Sacoolas was originally cooperating in the case, police have said, but she left the country despite assuring them she would remain in the UK.
The U.S. State Department confirmed Saturday in an e-mailed statement that the crash involved "a vehicle driven by the spouse of a USA diplomat assigned to the United Kingdom" and extended "deepest sympathies" to Dunn's family.
Police in Northamptonshire said Saturday, Oct. 5 they had been treating an unidentified 42-year-old woman as a suspect and that she had indicated she didn't plan to leave Britain. The rider of the motorcycle, 19-year-old Harry Dunn, was taken to a hospital in the nearby city of Oxford, where he died soon after, according to press releases from Northamptonshire Police. We're broken, inside and out.
"All we need to do is to ask her to come back".
She added: "We have to get proper justice for Harry and closure for his family".
Sacoolas claimed diplomatic immunity after the accident and she was allowed to avoid prosecution and leave England.
"(They) are considered carefully given the global impact such decisions carry; immunity is rarely waived".
Responding to a question on Twitter on Sunday, Mr Adderley confirmed that he and Stephen Mold, Police Fire and Crime Commissioner for Northamptonshire, had contacted the embassy for immunity to be waived.
"I do not think that it can be right to use the process of diplomatic immunity for this type of objective", the prime minister added.
Dunn is described as motorbike enthusiast who "lived his short life to the fullest" in a GoFundMe page called "Justice 4 Harry" that was started by a family friend. "We will do what we can to bring her back", she said.