Ministers were warned two days after the death of Harry Dunn that the ex-CIA agent responsible only had an ‘ambiguous’ claim to diplomatic immunity, The Mail on Sunday can reveal.

But after talks with Washington DC, the Foreign & Commonwealth Office (FCO) failed to pass on these concerns to police investigating American Anne Sacoolas for death by dangerous driving after she collided with the 19-year-old motorcyclist while driving on the wrong side of the road.

Nor did they tell Northamptonshire Police that the US had refused to waive her claim of immunity until after she had fled Britain two weeks later, despite being warned she was leaving.

Guidance from the Crown Prosecution Service states that it is a matter for the police to establish if a suspect of a crime has immunity rather than the Government.

Harry Dunn was killed when his motorbike crashed into a car outside RAF Croughton in Northamptonshire on August 27 last year

The Northamptonshire force is set to argue in court that this delay stopped them seeking powers to arrest Ms Sacoolas after the accident outside the US spy base RAF Croughton on August 27.

Two senior FCO Ministers, Heather Wheeler and Chris Pincher, were told by civil servants on August 30 that there was ‘some ambiguity around the case’, with the warning sent to Foreign Secretary Dominic Raab’s private office and FCO permanent secretary Sir Simon McDonald.

A well-placed source said the officials warned that the US government appeared to be exploiting a dubiously worded treaty to recuse Ms Sacoolas from criminal liability.

But three days later, on September 2, it is understood the FCO – following conversations with the US government – contacted Northamptonshire Police to say they believed Sacoolas had full immunity from prosecution.

The Northamptonshire force is set to argue in court that this delay stopped them seeking powers to arrest Ms Sacoolas (pictured) after the accident outside the US spy base RAF Croughton on August 27

On Saturday the force did not deny that they had failed to make any further investigation into the diplomatic situation at that point. These crucial days after the tragic crash are to be examined by the High Court after Harry’s Dunn’s family took legal action against the FCO and Northamptonshire Police.

Last month the MoS revealed that senior diplomat Neil Holland had texted a US official on September 15 to suggest Sacoolas ‘getting on the next flight out’. The police were not informed she had left until the following day.

Last night Harry’s mother Charlotte Charles said the latest revelations made her ‘more determined now than ever to bring the case to court and have this all exposed and those responsible for this mess held to account’.

Harry’s family are pursuing a judicial review to have Ms Sacoolas’s claims of immunity rendered unlawful, with senior Tories worried the FCO is on course for an embarrassing defeat.

One said: ‘Raab had been in the job for a matter of weeks so it’s not clear he had a full grip on the place yet. The more that comes out the worse this looks.’

A diplomatic source said: ‘It’s all kicking off between the FCO and the police.

‘The FCO appears to have told them she had diplomatic immunity and the police took that as gospel, when actually it was their job to investigate it.’

Chairman of the Foreign Affairs Select Committee Tom Tugendhat told the MoS: ‘Harry Dunn’s family are rightly concerned to know exactly what happened but we all need to know how immunity is used in our country. This has raised questions I know the Foreign Office will want to clarify as soon as possible.’

On Saturday a spokesman for Northamptonshire Police said ‘it would ‘be inappropriate to comment at this time.’

Speaking on behalf of Harry’s family, Ms Charles added: ‘We will not let what happened to us happen to anyone ever again and There have clearly been very serious failings at the FCO.’

The FCO said: ‘We are confident that we have acted properly and lawfully in relation to Harry’s death.’