After days of controversy surrounding the case, a letter which has been widely reported in the press from Dominic Raab to Harry's family, seen by the BBC on Saturday, has suggested that her return to the United States of America is reportedly rendered her diplomatic immunity status irrelevant.
It comes as lawyers for Anne Sacoolas, 42, who was granted diplomatic immunity and then fled back to the U.S. has said they will meet Harry's parents when they arrive today.
Mrs Sacoolas, the wife of a U.S. diplomat, has previously said she wants to meet the pair to "express her deepest sympathies and apologies".
It is alleged Sacoolas hit Harry head-on after she left the Northamptonshire base where her husband Jonathan worked.
The teenager's parents said they would like to take up the offer to meet Mrs Sacoolas when they land. "That is our red line in the sand", Dunn family spokesman Radd Seiger told Sky News.
Before the letter was sent by the FCO, the family's lawyer Mark Stephens told PA: "There are approximately 20,000 official diplomats in this country - there's a definitive list of who is and who isn't".
'We know definitively that this guy was not a diplomat and therefore was not entitled to diplomatic immunity.
Mrs Sacoolas "will continue to cooperate with the investigation", her lawyer added.
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'A statement from her lawyer is promising that we may be able to hopefully get a meeting put together - whether it's face to face or lawyer to lawyer, not really sure on that basis yet but fingers crossed we're stepping in the right direction'. "She would not have been entitled to claim diplomatic immunity".
They said they will visit both NY and Washington DC to gain support from Americans and "put pressure on the USA administration to do the right thing".
In a statement obtained by ITV News, a lawyer representing Mrs Sacoolas said: "Anne is devastated by this tragic accident".
Mrs Charles said: 'The letter from the FCO was incredible, we felt like we finally had a breakthrough, we finally had confirmed that the immunity that we didn't think she had has been confirmed, that she doesn't have it, certainly since she absconded back to the USA.
'No loss compares to the death of a child and Anne extends her deepest sympathy to Harry Dunn's family'.
Harry Dunn's mother has said "sorry doesn't cut it" ahead of a meeting with the woman involved in a vehicle crash which killed her son. "They committed right from the start that she needs to come back and present herself to Northamptonshire police and then let the process go where it may".
Sacoolas' departure prompted Dunn's family to appeal for justice and British Prime Minister Boris Johnson to raise the issue with President Donald Trump.