The Prime Minister has joined the growing number of calls for the US to revoke its decision to grant diplomatic immunity to the suspect in the death of Northampton Town fan Harry Dunn.
Foreign Secretary Dominic Raab, the chief constable of Northamptonshire Police and many others have urged the American woman, now named as Anne Sacoolas, to return to the UK.
During a visit to a hospital in Watford, Boris Johnson said: "I think everybody's sympathies are very much with the family of Harry Dunn and our condolences to them for their tragic loss.
"I must answer you directly, 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 law as they are carried out in this country.
"That's a point that we've raised or are raising today with the American ambassador here in the UK and I hope it will be resolved very shortly.
"And to anticipate a question you might want to raise, if we can't resolve it then of course I will be raising it myself personally with the White House."
Harry died in hospital following the collision on the B4031 Park End outside Croughton, near Brackley, on August 27.
The 19-year-old, from the nearby village of Charlton, was riding a motorcycle at the time of the head-on collision with a car.
Police confirmed on Saturday that a 42-year-old American woman is being treated as a suspect but has left the country after previously stating she had no plans to.
Mrs Sacoolas has apparently been granted diplomatic immunity as she is the wife of a US diplomat - it has been reported she was based at RAF Croughton, which is used by the US Air Force.
A Foreign and Commonwealth Office (FCO) spokesman said Mr Raab has called the US ambassador to express the UK’s disappointment with their decision, and to urge the embassy to reconsider this decision.
"The Foreign Secretary has agreed to meet the family and details of the meeting are being finalised," she said.
"Andrea Leadsom, the family’s local MP, has also met the family. FCO officials are in contact with the family through their lawyer and Northants Police."
Many people on social media have tweeted US President Donald Trump in a bid to get her to come back to the UK to face justice.
But the US State Department has said diplomatic immunity was 'rarely' waived despite the pressure from British officials.
The PM's spokesman said: "The details of this case are extremely concerning and the Foreign Secretary has discussed it with the US ambassador.
"The justice process should be allowed to take place and we urge the US to reconsider their decision.
"The Prime Minister offers his condolences to the Dunn family - this was a tragic set of events that no family should have to experience."