British prosecutors have charged the wife of a US diplomat with causing death by dangerous driving and will seek her extradition over a car crash in which a teenager died.

Key points: Harry Dunn died after a collision between his motorcycle and a car driven by Anne Sacoolas

Harry Dunn died after a collision between his motorcycle and a car driven by Anne Sacoolas She cooperated with police after the crash, but later claimed diplomatic immunity and left the United Kingdom

She cooperated with police after the crash, but later claimed diplomatic immunity and left the United Kingdom The British Government has not yet made a formal request to extradite Ms Sacoolas

Harry Dunn, 19, died after his motorcycle collided with a car driven by Anne Sacoolas in August near RAF Croughton, an air force base in Northamptonshire in central England which is used by the US military.

Ms Sacoolas was given diplomatic immunity and left Britain shortly after the accident, setting off a dispute between London and Washington over whether she should return to face investigation.

Loading

"The Director of Public Prosecutions has met with Harry Dunn's family to explain the basis of the decision we have made following a thorough review of the evidence available," the United Kingdom's Crown Prosecution Service (CPS) said in a statement.

But it said it was up to the Home Office to decide whether to seek Ms Sacoolas's extradition formally through diplomatic channels.

Dunn's mother, Charlotte Charles, broke down in tears after finding out about the charges, saying it meant she had kept a promise to her son to get him justice.

"We had no idea it was going to be this hard and it would take this long, but we really do feel it is a huge step towards that promise to Harry," she told reporters.

British Foreign Minister Dominic Raab welcomed the charging decision, adding in a statement: "I hope that Anne Sacoolas will now realise the right thing to do is to come back to the UK and cooperate with the criminal justice process."

In a statement, the State Department expressed "deepest sympathies" over Dunn's death, but said the decision to charge Ms Sacoolas was not "a helpful development".

"We are disappointed by [the] announcement and fear that it will not bring a resolution closer," a State Department spokesperson said.

"The United States has been clear that, at the time the accident occurred, and for the duration of her stay in the UK, the driver in this case had status that conferred diplomatic immunities."

Sacoolas refuses to return to UK, lawyer says

Loading...

In a statement, Ms Sacoolas's US lawyer Amy Jeffress said her client would "not return voluntarily to the United Kingdom to face a potential jail sentence for what was a terrible but unintentional accident".

Dunn's case gained international prominence when his parents met US President Donald Trump at the White House in October, an occasion he described as "beautiful" but "sad".

Mr Trump hoped to persuade them to meet Ms Sacoolas, who was in the building at the same time, but they declined.

Radd Sieger, a spokesperson for the family, said that US national security adviser Robert O'Brien had "quite emphatically" said that Ms Sacoolas would not be "coming back" during the meeting.

Harry Dunn (pictured) was killed in August. ( Facebook )

The Dunn family has continued to press Ms Sacoolas to return to Britain to face police questioning about the crash.

In December, she was approached by a journalist from British broadcaster ITV in the US but refused to answer questions about whether or not she would return to the UK.

She initially cooperated with local police after the crash, but later claimed diplomatic immunity.

However, British legal counsel acting for the Dunn family is challenging whether Ms Sacoolas held the diplomatic immunity that allowed her to leave the UK in the first place.

Loading

Under the 1961 Vienna Convention, diplomats and their families are immune from persecution but that immunity can be waived by the state that has sent them.

But British prosecutors argue that immunity does not apply to dependents of consular officials based outside of London.

The maximum jail sentence in Britain for causing death by dangerous driving is 14 years.

The White House and the US Department of Justice did not immediately respond to a request for comment.

ABC/Reuters