Harry Dunn's brother says diplomatic immunity granted to Anne Sacoolas was 'like a punch in the face'

Charlotte Charles with her son Harry Dunn who died after his motorbike collided with a car near RAF Croughton in Northamptonshire on August 27. (PA)

Harry Dunn’s twin has described Anne Sacoolas’s return to the US under diplomatic immunity following his brother’s death as like “a punch in the face”.

Niall Dunn has been house-bound with grief since his brother was killed eight weeks ago in a car crash outside RAF Croughton in Northamptonshire, but has now appealed directly to the US diplomat’s wife to “cooperate and help us”.

"When we were told, it was like a punch in the face really, telling you: ‘Tough, she's not coming back, end of story, go and cry at home’,” he said.

"That's really what it did feel like. I’m not happy with how everything has happened,” Niall told the Mail on Sunday.

Floral tributes lay on the roadside near RAF Croughton in Northamptonshire where Harry Dunn died. (Getty)

Since the fatal collision, Niall’s parents Charlotte Charles and Tim Dunn have launched an international campaign to bring Ms Sacoolas, the wife of a US diplomat, to trial.

Harry’s parents have met with US President Donald Trump in Washington to discuss the matter.

But there is no indication Mrs Sacoolas’s diplomatic immunity will be waived for her to be forced to return to the UK to face an investigation.

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“It was swept under the carpet and that’s what really got to me. It made me angry. Harry lost his life. We’ve lost everything. I want to know who decided this was OK to do this to us,” Niall added.

Appealing to Mrs Sacoolas, Niall said: “We want for this to be over, to live our lives the best way we can. We can’t do that unless you cooperate and help us.

“This all could have been avoided if you had just stayed here and done the right thing. Although that time’s passed you can still put an end to all of this.”

Earlier this week, lawyers representing Harry Dunn’s family announced they would be taking legal action against the Foreign Office and launching an investigation into police conduct “imminently”.

The teenager’s family spokesman, Radd Seiger, told the PA news agency they would be referring Northamptonshire Police to the Independent Office for Police Conduct (IOPC) over their actions during the investigation into his death.

The lawyers have begun the process of drafting letters; one called a “letter of claim”, which is a prelude to a judicial review, about the advice given to Northamptonshire Police by the Foreign Office, and the other to request an investigation by the IOPC into the conduct of the force.

Mr Seiger said this was “just the beginning” and would also, at a later stage, review the role of the US administration in the granting of diplomatic immunity to the American suspect, Anne Sacoolas.