A US diplomat's wife suspected of causing the death of a teenage boy in a hit-and-run claimed diplomatic immunity to flee the UK.

The woman is being investigated in connection with the death of Harry Dunn, 19, who was killed when his motorbike collided with a car near RAF Croughton in Northamptonshire in August.

Police believe that she pulled out of the airbase onto the wrong side of the road before colliding head on with Mr Dunn, according to reports.

Mr Dunn suffered multiple injuries and died later on in hospital.

View photos Tragic: The 19-year-old was on his motorbike when a car believed to be driven by the diplomats wife collided with him. (Facebook) More

The teenager's family said they are “utterly broken” and have appealed to President Trump to help them in their fight for justice.

In a statement to Sky News, they said: "President Trump, please listen. We’re a family in ruin. We’re broken.

"We can’t grieve. Please, please let her get back on a plane."

Sky News reported that the alleged driver is the wife of a US diplomat who was reportedly advised to leave the UK by the American Embassy.

An attempt by Northamptonshire Police to stop her leaving was declined - and the US Embassy has confirmed she's no longer in the UK.

View photos Harry's family have now appealed to President Trump to send her back to the UK. (Facebook) More

READ MORE FROM YAHOO NEWS

US diplomat's wife suspected of involvement in fatal road crash left UK

Schoolboy, 14, left for dead after being launched into the air by 60mph hit and run driver

Surveillance video shows teen being dragged by hit-and-run driver's car in South Los Angeles

Mr Dunn's mother Charlotte Charles said: "Everyone loved him, we're utterly broken inside and out.

"Everything hurts day and night, it's an effort to do anything, I ache from it, every limb, every internal organ hurts."

Foreign Secretary Dominic Raab has called the US ambassador in London to ask them to reconsider.

"I wish to offer my condolences to the family affected by this tragic incident.

"I have called the US Ambassador to express the UK's disappointment with their decision and to urge the Embassy to re-consider it."

Northamptonshire police said the force was “exploring all opportunities” to ensure that its investigation continued.

Superintendent Sarah Johnson told Sky News: "We had met with the suspect in the presence of legal advisers in order to identify the best way forward."

"During the course of that meeting we were aware that diplomatic immunity had been raised as an issue, we then made immediate application for a waiver in order for us to undertake further investigations and interview.

"Unfortunately we were latterly advised that the waiver had been declined and the suspect had left the UK."