Video report by ITV News Correspondent Neil Connery
Harry Dunn's mother has called on Dominic Raab to "reevaluate his position" after ITV News learned a Foreign Official official said there was "not much mileage" in keeping his alleged killer and her family in the UK just three weeks after his death.
Mr Dunn was killed when his motorbike was involved in a collision with a car allegedly driven by Anne Sacoolas, the wife of a US intelligence officer on August 27, 2019.
Ms Sacoolas left the UK around three weeks after the crash, which took place near RAF Croughton in Northamptonshire.
In the days after Mr Dunn's death, Northamptonshire Police said the Foreign Office told them Ms Sacoolas had diplomatic immunity and they would seek a waiver.
Over the following fortnight, Northamptonshire Police said they were not informed about the legality of Ms Sacoolas's immunity while she was still in the UK.
On Friday September 13, two senior Foreign Office officials met US embassy staff who said diplomatic immunity would not be waived and that the Sacoolas family would return to the US unless the UK had strong objections - something which officials say they expressed.
However just a day later, on Saturday, September 14, a senior Foreign Office official sent a text to their US embassy counterpart, which read: "I think that now the decision has been taken not to waive, (immunity) there's not much mileage in us asking you to keep the family here.
"It's obviously not us approving of their departure but I think you should feel able to put them on the next flight out..."
Mr Dunn's mother Charlotte Charles says she can 'never' trust Foreign Secretary Raab again
On Sunday September 15, Ms Sacoolas and her family flew back to America.
The US government refuses to extradite her back to the UK.
Northamptonshire Police said they were only informed she had left the following day.
Documents seen by ITV News reveal the Senior Investigating Officer recalls being told Ms Sacoolas had contacted Foreign Office officials days before leaving asking if there was anything stopping her and her family from going.
Charlotte Charles, Harry's mother, said: "It's just beyond belief that given the number of times we've with met them.
"We should have been properly grieving in those first few months.
"We should have been given the chance and we deserved the truth.
"To send that in a text message... I mean seriously who the hell have we got running our country?"
'He deserves the truth': Mr Dunn's father speaks out after new claims against Foreign Office
ITV News has also seen a Foreign Office briefing note copied to Mr Rabb's Private Secretary sent three days after the crash outlining what happened.
It reveals concern over the potential for "some very unpalatable headlines".
Asked whether they could ever trust Mr Raab again, Ms Charles said: "Never, nor his team.... I'll never trust any of them again.
"As far as I'm concerned he needs to completely re-evaluate the position he holds."
Harry's father, Tim Dunn, said: "I felt from the start they lied and they've messed it up.
"We must get the truth for Harry's sake... he was a good lad... and he deserves it."
Radd Seiger, Harry Dunn's family lawyer, said: "It is now crystal clear that the Foreign and Commonwealth Office [FCO] let Harry’s family down badly after he died.
"There is now no doubt about it.
"Serious questions will have to be asked.
"In not passing relevant information and material about the ambiguity of Mrs Sacoolas’ diplomatic position to Northants Police, who’s responsibility it was to establish if she had diplomatic immunity, the FCO interfered in the police's investigation into the case."
He added: "It is high time the FCO come clean, admit their conduct, drop their absurd defence to the Judicial Review and approach us to discuss openly and with transparency what they are going to do to clear this mess up for the benefit of the family.
"It is the least they deserve after all they have been through."
In a statement, Northamptonshire Police said: “Charlotte Charles and Tim Dunn are presently seeking to bring a judicial review against the Secretary of State for Foreign and Commonwealth Affairs and the Chief Constable of Northamptonshire Police.
"It would therefore be inappropriate for Northamptonshire Police to comment further at this time.”
In a statement to ITV News, the Foreign Office said: “We consistently called for Anne Sacoolas’s immunity to be waived before she left the UK.
"Both the prime minister and the foreign secretary have been clear with the US that the refusal to extradite her amounts to a denial of justice, and that she should return to the UK."
Shadow foreign secretary Lisa Nandy said: "This new evidence shows why a parliamentary inquiry is really, really needed.
"You've got essentially a young man who lost his life and no one has been held to account for that, Justice has just not been done. With fresh evidence that protocol may not have been followed it's really important now that the family start to get some answers."