President Joe Biden is 'actively engaged' and 'extremely sympathetic' in the case of motorcyclist Harry Dunn, Boris Johnson said following their first in-person meeting.
The leaders discussed the 19-year-old, who was killed when a car crashed into his motorbike outside a military base in Northamptonshire in August 2019, ahead of the G7 summit of world leaders in Cornwall.
Mr Johnson said President Biden has 'his own personal reasons for feeling very deeply about the issue,' adding he was 'extremely sympathetic' during their discussion.
The death of Harry sparked an international controversy after suspect Anne Sacoolas had diplomatic immunity asserted on her behalf by the US Government.
Sacoolas, the wife of a US intelligence official based at RAF Croughton, has since been charged with causing the teenager's death by dangerous driving.
Harry Dunn, 19, was killed when a car crashed into his motorbike in August 2019
Boris Johnson and President Joe Biden discussed the case in their first face-to-face meeting
The Prime Minister was asked if there had been any progress made on the case following his first face-to-face meeting with President Biden yesterday.
Speaking to the BBC, Mr Johnson said: 'You should really - when you get the chance - put your question to the president because he is actively engaged in the case.
'As you know, he has his own personal reasons for feeling very deeply about the issue. And he was extremely sympathetic, but this is not something that either government can control very easily because there are legal processes that are still going on.
'But he did express a great deal of sympathy, as indeed this government continues to do for the family of Harry Dunn.'
He added both the US executive and legal and judiciary system were 'working together'.
Speaking on Radio 4's Today programme, Harry's mother Charlotte Charles said she would welcome the opportunity to meet President Biden after their encounter with Donald Trump 'clearly didn't go well.'
She added: 'We had a pretty nasty trick played on us that has left some quite deep emotional scars to be honest. It was one horrific day.
'So absolutely, we would very much welcome a meeting with President Biden, considering he has got personal experience of what we are going through.'
Anne Sacoolas, 43, had diplomatic immunity asserted on her behalf by the US Government and was able to return to her home country 19 days after the collision
The Dunn family said they were pleased to see the case raised at the 'first available opportunity'. Pictured: Harry's parents Charlotte Charles and Tim Dunn
The Dunn family earlier said they were pleased to see Harry's case raised at the 'first available opportunity'.
Speaking yesterday, family spokesman Radd Seiger said: 'Harry's parents are very pleased to see that the PM has taken the opportunity to raise the case with President Biden at the first available opportunity.
'This rightly shows just how important this issue is and we are very grateful to the Prime Minister and his team for doing so.
How was Anne Sacoolas able to flee back to the US as British police investigated?
Anne Sacoolas used a loophole to claim diplomatic immunity after allegedly killing Harry Dunn in a crash involving her Volvo SUV.
It was believed that diplomatic immunity only applied to US officials - and their families - if they worked at the US Embassy in London.
But it appears that because of the work done at RAF Croughton in Northamptonshire – a US intelligence hub in Britain - the same applies there.
Sky News claims an immunity deal between the UK and US there started in 1994.
As a result Northamptonshire Police were planning to get Anne Sacoolas to sign a 'waiver of diplomatic immunity' - but she, her husband and their children fled on a private jet.
Harry's family claim that Mrs Sacoolas had promised to work with police and admitted culpability.
'As all parties know there is no greater force on earth than the love between a mother and child. The family will continue to pursue justice until it is done.'
The Dunn family have challenged the diplomatic immunity asserted on Sacoolas's behalf, which will be heard in the Court of Appeal next year.
Harry's parents, Ms Charles and Tim Dunn, have also brought a civil claim against the suspect and her husband in the US state of Virginia.
Ms Charles today recalled her family's meeting with former US president Trump in October 2019, when she was informed that Anne Sacoolas was in a nearby room.
She said: 'We shouldn't have been shocked with Mr Trump's record prior to that.
'But it's still a massive shock when you are still in shock from having lost your son.
'We had only lost Harry seven or eight weeks beforehand. To then be told all of a sudden that she was in the room next door... it wouldn't have been good for us.
'We weren't even aware that we were seeing President Trump.
'We rocked up at the White House, having received the invitation. We were just told that we were going to see a senior official.
'Not for a second did we think that it would be Mr Trump... we had to just roll with the punches like we always do and take it in our stride and just face everything head-on, which seems to be pretty much our lives over the last 22 months.'
She added it meant a 'tremendous amount' that Mr Johnson and President Biden had discussed her son's case at the earliest possible opportunity.
'We couldn't be more grateful, it means a lot to us,' Ms Charles said.
'It's good to hear that it's being discussed at the highest of levels but we are clearly anxious to await further information to see exactly where the comments made yesterday will lead to.
The Prime Minister was asked if there had been any progress made on the case following his first face-to-face meeting with President Biden yesterday. Pictured: Mr Johnson and President Biden in Cornwall yesterday
Speaking to the BBC, Mr Johnson (above) said: 'You should really - when you get the chance - put your question to the president because he is actively engaged in the case'
'My hope, as always, my family's hope as always, is to ensure that I can complete my promise to Harry. We will not rest until justice is done.'
Asked whether a civil claim lodged in the US could represent justice for Harry, if the US continues to assert suspect Sacoolas' diplomatic immunity, Ms Charles added: 'No. Justice means standing up and accepting accountability for your actions.
'The civil claim is extremely separate to the criminal case.
'So justice still needs to be done here in the UK. That's always been my goal - that isn't going to change.
'She needs to face UK jurisdiction, it has to be done.'
WHY ARE AMERICANS AT RAF CROUGHTON IN THE UK?
RAF Croughton is an air base that is currently being leased by the US government.
It houses the 422nd Air Base Group, but is also being used by spies working for the Joint Intelligence Operations Center Europe (JIOCEUR).
JIOCEUR is a military intelligence analysis center which is part of the Defense Intelligence Agency. The agency is an external branch of government which provides intelligence to 'warfighters, defense policymakers and force planners in the Department of Defense and the Intelligence Community'.
The entrance to RAF Croughton is shown. Sacoolas was exiting the base when she turned onto the wrong side of the road on August 27
It provides intelligence information for the U.S. European and African commands as well as NATO.
The Center is based at RAF Molesworth in Cambridgeshire but, following the 2015 announcement that it was to close in 2023, many of the positions were moved to Croughton.
There are plans to consolidate it with the U.S. Africa Command to make a larger station at Croughton that will be known as the Joint Intelligence Analysis Complex - a major hub for US intelligence gathering.
According to locals in Croughton, the communications center - where Sacoolas' husband is said to work - is a 'site within the site' which has its own separate security.
The US government is reconsidering the relocation after being met with resistance from lawmakers who said it would be too expensive.
A file photo of a geodesic dome covering radar scanners and satellite dishes at the base. It is an intelligence gathering hub which the US Defense Intelligence Agency uses to collect information from Europe and Africa