Boris Johnson is "hopeful" progress can be made in returning a suspect from the United States in the case of teenage motorbike crash victim Harry Dunn.
But the Prime Minister sounded a note of caution as he insisted progress is "by no means a certainty" when asked if he would be pressing US President Donald Trump for the return of Anne Sacoolas.
Harry, 19, was killed when his motorbike crashed into a car outside RAF Croughton in Northamptonshire on August 27.
Mrs Sacoolas, the 42-year-old motorist allegedly responsible for the crash, claimed diplomatic immunity and was allowed to return to the US.
Mr Johnson told reporters during an election campaign visit to Wiltshire: "On Anne Sacoolas, as you may know I have intervened with the president already on that I think on a couple of occasions, and we are hopeful that progress can be made but I wouldn't want to go further than that."
Outside the palace, Harry's mother Charlotte Charles spoke to reporters about her hopes of discussing her son's case with Mr Johnson and Mr Trump.
Harry's family brought their case to Mr Trump at a visit to the White House in October, with Mrs Charles describing the president as "gracious" and "welcoming".
She added: "But it was quite evident within a couple of minutes of being in the Oval Office that he wanted us there to try and smooth things over as such and have us meet with Anne Sacoolas who was in the next room.
"He was probably hoping that we would just then go away and grieve."
Mrs Charles said it was "upsetting" that the UK and the Queen were hosting Mr Trump while he was "harbouring" the suspect in Harry's case.
Asked if she trusted the political leaders to discuss the case, she said: "We don't really trust any of them at the moment.
"We've been lied to I think on a few occasions... it's extremely hard to trust anybody."
- Harry parents express 'misery' at prosecuting decision
Earlier on Tuesday, a government spokesman said Foreign Secretary Dominic Raab, who has been criticised by the Dunn family for his handling of the case, raised the issue with US Secretary of State Mike Pompeo when they met in London.
Mrs Charles said she had received no information on the meeting and criticised the Government's response to Harry's case as "very poor".
She called on authorities to better communicate with the family over progress made on the case.
"Until they do, we're going to continue to be suspicious that something else is going on behind the scenes, so I would just ask them to come and talk to us," she added.
It is understood that Harry's parents have also expressed their "misery" to the Crown Prosecution Service (CPS) over the time it has taken in deciding whether or not to charge Mrs Sacoolas.
The CPS has said it is "doing everything possible to reach a charging decision as soon as we can".
Addressing the issue during his visit to London, Mr Trump repeated the US position that Mrs Sacoolas had diplomatic immunity but said he had also met her, adding: "We're trying to work something out."