The mother of Harry Dunn has called for the Foreign Secretary’s resignation over his department’s handling of her son’s death.

Charlotte Charles said the way the Government had dealt with her family was “beyond words”, adding: “You wouldn’t treat an animal the way Government treated us.”

Documents seen by the PA news agency showed that a senior diplomat at the FCO had sent a text message to their US Embassy counterpart saying they should “feel able” to put suspect Anne Sacoolas on the next flight home.

Initial disclosure documents, which surfaced earlier this year, showed a briefing note copied to Dominic Raab’s private secretary from three days after the fatal crash – which revealed concern for some “very unpalatable headlines”.

A witness statement from the senior investigating officer at Northamptonshire Police said an official at the FCO had requested the force delay telling the family that a waiver for Mrs Sacoolas’s diplomatic immunity had been declined by the US – adding that it would help if they could get their “ducks in a row” beforehand.

Mr Dunn was killed when his motorbike crashed into a car outside a US military base in Northamptonshire on August 27 last year.

Anne Sacoolas, 42, the wife of a US intelligence official based at RAF Croughton, claimed diplomatic immunity following the crash and was able to return to her home country.

She was charged with causing death by dangerous driving in December, but an extradition request for Sacoolas submitted by the Home Office was rejected by US Secretary of State Mike Pompeo in January – a decision later described by the State Department as “final”.

Mrs Charles was prompted to call for Mr Raab’s resignation after reports from ITV News showed pictures of the Foreign Secretary at a reception, hosted by the US Ambassador, three days before Sacoolas left the country.

Addressing her reasons for calling for Mr Raab’s resignation, Mrs Charles said: “Any parent out there who has lost a child will understand what we are going through as a family.

“The lack of sleep. The continual pain in the stomach. That constant realisation that Harry is not here.

“The unbearable feeling that he will never walk through the door again and give me one of his hugs. Never call or text again. Not watching him going into what would have been the prime of his life.

“But as if all that was not enough, what on earth did this government think it was doing treating us the way they did? It is beyond words.

“You wouldn’t treat an animal the way government treated us.

“Every single one of those responsible must go. I don’t care how high this goes.”

Mrs Charles continued: “This must never happen again. Let us be the last ones. I wouldn’t wish this on my worst enemy.

“Mr Raab has to go but we will not stop until we get to the bottom of all this, find out who was involved and make sure that anyone else who should not be in their job as a result also resigns or is dismissed.”

A Foreign Office spokeswoman said: “We have the deepest sympathy for Harry’s family. No family should have to experience what they have gone through.

“The case remains of the highest priority for the Foreign Secretary who continues to raise the case with the US Government, including last week.

“Both the Foreign Secretary and the Prime Minister have been clear with the US that the refusal to extradite Anne Sacoolas amounts to a denial of justice, and that she should return to the UK.

“We are committed to revising the arrangements at RAF Croughton to ensure they cannot be used in this way again.

“The Foreign Secretary remains ready to meet Harry’s family and to support them to get the justice they deserve.”