Harry Dunn’s parents’ High Court battle against the Foreign Office, which they claim unlawfully obstructed the police investigation into their son’s death, is due to begin.

The 19-year-old was killed when his motorbike crashed into a car being driven on the wrong side of the road by American Anne Sacoolas outside RAF Croughton in Northamptonshire.

Sacoolas, whose husband Jonathan Sacoolas worked as a technical assistant at the base, left the country a few weeks later after the US asserted she was entitled to diplomatic immunity.

Here, the PA news agency looks at the circumstances surrounding the use of diplomatic immunity.

What is diplomatic immunity?

Diplomatic immunity is a legal exemption from certain laws granted to diplomats by the state in which they are working.

It ensures they will not be liable to be prosecuted under the host country’s laws. It is governed by an international treaty called the Vienna Convention on Diplomatic Relations and UK legislation called the Diplomatic Privileges Act.

What would have to happen in order for someone who has been granted diplomatic immunity to face prosecution?

The only way for somebody who has been granted diplomatic immunity to face prosecution would be for their country of employment to waive their immunity.

In the case of Harry Dunn, the US would have to waive Anne Sacoolas’s immunity if she was to face prosecution in the UK.

The US State Department has declined to waive Sacoolas’s immunity on a number of occasions and has rejected an extradition request submitted by the Home Office.

Can Anne Sacoolas waive her own diplomatic immunity?

The Crown Prosecution Service have recently written to Anne Sacoolas’s legal representatives “explaining that the next step in this case is for her to surrender to the jurisdiction of the court”.

It is understood her lawyers believe it is for the US Government to waive Sacoolas’s immunity, not Sacoolas herself.

Why was diplomatic immunity asserted on Anne Sacoolas’s behalf?

The US Government and the Foreign, Commonwealth and Development Office’s (FCDO) position is that dependents (such as spouses or children) of US administrative and technical staff at RAF Croughton in Northamptonshire have diplomatic immunity.

Can diplomatic immunity be asserted on behalf of a US administrative and technical staff member at RAF Croughton?

Due to an “Exchange of Notes” between the UK and the US in 1995, administrative and technical staff at RAF Croughton are entitled to diplomatic immunity for actions performed in the course of their professional duties under the Vienna Convention.

For any actions outside of their professional duties, their immunity is pre-waived upon their arrival in the UK.

Why does the FCDO believe Anne Sacoolas had diplomatic immunity?

The Exchange of Notes which extends diplomatic immunity to US administrative and technical staff at RAF Croughton does not explicitly mention whether or not dependents must have their diplomatic immunity pre-waived.

The FCDO argued at a preliminary hearing in June that, because Article 32 of the Vienna Convention says a waiver of diplomatic immunity “must always be express”, the absence of a reference to dependents in the Exchange of Notes means Sacoolas, as the wife of a member of administrative and technical staff at RAF Croughton, did not have her diplomatic immunity pre-waived on her arrival in the UK.

Why do the Dunn family believe Anne Sacoolas did not have diplomatic immunity?

The Dunn family’s lawyers will argue that under international law, just as a waiver of diplomatic immunity is not express for dependents in the Exchange of Notes, the immunity itself was not express for family members.

They will assert that because dependents are not mentioned in the Exchange of Notes whatsoever, Anne Sacoolas did not have diplomatic immunity.