The family of Harry Dunn flew to the US yesterday for a potential showdown with the diplomat's wife accused of killing the teenager in a car crash.

Harry's parents Charlotte Charles and Tim Dunn hailed a 'breakthrough' after learning US citizen Anne Sacoolas no longer has diplomatic immunity from potential prosecution over the case.

But the family insist they will only meet the US woman suspected of causing their son's death if she promises to return to Britain.

Family spokesman Radd Seiger told Sky News the condition was a 'non-negotiable red line in the sand' if Anne Sacoolas wished to meet with the teenager's parents while they are in America.

The Foreign Office made the revelation in a letter to the family yesterday, explaining it is because her husband Jonathan has left his posting at RAF Croughton in Northamptonshire.

Harry Dunn's parents, Charlotte Charles and Tim Dunn (pictured) hailed a 'breakthrough' after learning that US citizen Anne Sacoolas no longer has diplomatic immunity

Mrs Charles said: 'The letter from the FCO was amazing, 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.

'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.'

Mrs Sacoolas's lawyers released a statement at the weekend describing the death of Harry, 19, in August as a 'tragic accident'. They said she is 'devastated' and has expressed a desire to meet with the teenager's family, who have arrived in New York in their quest for justice.

Harry Dunn, 19, (pictured) was killed after Sacoolas, 42, crashed her Volvo SUV into him near RAF Croughton in Northamptonshire

Harry's parents are planning to give a series of interviews with America's main TV networks to heap pressure on the US government to hand Mrs Sacoolas over.

Sacoolas (pictured) initially claimed diplomatic immunity to avoid potential prosecution over the case

They will then travel to Washington DC to meet senior figures in the US government to express their outrage at the handling of the case.

Their lawyer Radd Seiger told the Mail: 'Harry's parents want to look the US President in the eye and ask him to resolve this painful situation. He needs to understand they are utterly heartbroken.

'We will not rest until we have Mrs Sacoolas back in the UK. That's the only way they can get closure.' Mr Seiger said Mrs Sacoolas, 42, has been asked four times if she would be willing to return to the UK, and on each occasion she failed to respond to the question.

He added: 'Harry's family just want a direct answer as to whether she will to back to the UK and continue to help the police with their investigation.

Mrs Charles (pictured) said: ''The letter from the FCO was amazing, 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'

'We've asked the same question four separate times and on each occasion this question is ignored. That's unacceptable.'

Mrs Charles, Harry's mother, said that Mrs Sacoolas's response to the crash 'just doesn't cut it'.

'My opinion on Anne Sacoolas now wanting to come forward and say sorry... is not really quite enough,' she told Sky News.

'But I'm still really open to meeting her, as are the rest of us. I can't promise what I would or wouldn't say, but I certainly wouldn't be aggressive.' Harry's parents are reluctant to meet the mother of three unless she gives some sort of assurances that she is willing to be extradited to the UK.

Police have CCTV of Sacoolas driving on the wrong side of the road (pictured are new signs that have appeared outside the RAF base) but she claimed diplomatic immunity, meaning detectives could not launch a criminal investigation

But the matter may now be taken out of her hands if the Crown Prosecution Service applies to the US to extradite her. As the wife of a US intelligence officer, Mrs Sacoolas initially claimed diplomatic immunity and fled the UK after the crash just outside the military base.

On Saturday evening the Foreign Secretary Dominic Raab wrote to Harry's family and said: 'The UK Government's position is that immunity, and therefore any question of waiver, is no longer relevant in Mrs Sacoolas's case, because she has returned home.

Mrs Sacoolas's lawyers released a statement at the weekend describing the death of Harry (pictured) in August as a 'tragic accident

'The US have now informed us that they too consider that immunity is no longer pertinent.' Although grateful for the letter, Harry's family want to know why the Foreign Office only reached this conclusion on the eve of their trip to the US – weeks after Mrs Sacoolas left the country.

Mr Seiger said: 'The family could have been told weeks ago that she no longer had diplomatic immunity. The failure to do so has put needless and unnecessary stress on a grieving family.'

Harry suffered horrific injuries in the crash and died later in hospital. Mrs Sacoolas had pulled out of the base, a US spy hub, on the wrong side of the road and collided with the teenager's motorbike on the brow of a hill.

New road markings and a sign have appeared outside the base. Arrows indicating the direction of travel have been painted on each side of the road and a yellow 'Please Drive on Left' sign has also been placed on the roadside.

Home Secretary Priti Patel yesterday played down suggestions that Mrs Sacoolas could be extradited from the US.

Asked on BBC1's The Andrew Marr Show whether this could happen, she said: 'It very much seems that the lady in question wants to start co-operating with the discussions and the investigations and I think that we should support that.

'We need to ensure that justice is done but obviously that co-operation with this investigation takes place. That is absolutely right.'