Lawyers acting for a teenager who died after a collision with a car allegedly driven by an American woman want the High Court to publish a secret document protecting her from prosecution.

Harry Dunn, 19, died last summer after his motorbike collided with a car near RAF Croughton in Northamptonshire. Anne Sacoolas, the wife of a US agent who worked at the base, has been charged with causing death by dangerous driving and has apologised to Dunn’s family, but is refusing to return to Britain.

US secretary of state Mike Pompeo has rejected the UK’s request for her extradition, claiming that at the time of the collision she had immunity from criminal prosecution, even though her husband did not.

The Foreign Office has confirmed the existence of a secret agreement between the UK and the US, signed in 1995, granting US staff working at the base diplomatic immunity, though they are still subject to criminal prosecution under UK law.

Foreign secretary Dominic Raab has told parliament that in his view an anomaly in the agreement meant that, while the US staff had waived their rights to be protected against criminal prosecution in the UK, this did not extend to their families. He has pledged to redraft the agreement after learning about the loophole.

Facebook Twitter Pinterest The Dunn family lawyers claim Foreign Secretary Dominic Raab has opposed their wishes to have a court clarify the law. Photograph: Daniel Leal-Olivas/AFP via Getty Images

But lawyers acting for the Dunn family said there were grave doubts over whether the anomaly existed or, if it did, afforded the legal protection Raab has claimed. They want the document setting out the secret agreement – which has been disclosed in the High Court, where the lawyers are challenging the immunity claim – made public.

“The Dunn family, backed by senior diplomats and international lawyers, say there is no such thing in international law as an ‘independent immunity’ for partners – their immunity is derived only from the diplomatic agent, and when the agent has none, nor does his wife or family,” said Mark Stephens, the family’s solicitor. “The US/Raab argument means that if a diplomat kills his wife, he can, under the 1995 agreement, be prosecuted for murder. If she kills him, however, she will be immune from prosecution.”

The Dunns’ legal team suggested the anomaly “could leave over 1,000 partners and friends of all the US agents in Britain entitled to commit criminal offences without trial or punishment”. It has been reported that there have been three cases of American vehicles being driven on the wrong side of the roads outside Croughton since Dunn’s death.

The agreement was apparently signed off as the US looked to bolster its spying operations at the Northamptonshire base.

“It would have been astonishingly negligent for Foreign Office lawyers to draft documents that created an anomaly which was not in the public interest, and negligent for the Foreign Office not to reveal this so that parliament could debate whether hundreds, perhaps thousands, of Americans should be free from punishment for any crime they might commit,” Stephens said. “Mr Raab not only insists that this is what was done by the previous government but opposes the Dunn family in their wish to have a court clarify the law. He has hired a number of expensive lawyers to argue in favour of the anomaly and has refused to help the Dunn family with their legal costs.”

Stephens said it was vital for a court to decide whether Sacoolas genuinely enjoyed immunity from prosecution at the time of her crime. “If she was not, that would pull the rug from Pompeo’s refusal to extradite her,” he said.

It is unclear how Sacoolas and her family were able to leave the UK in the days after the crash, but the US government insists it notified the Foreign Office that they were leaving.

There have been claims that she herself was a spy. The Foreign Office has said only that: “Anne Sacoolas was notified to us as a spouse with no official role.”