Boris Johnson has acknowledged that the chances of the US sending Anne Sacoolas to the UK to face justice over the death of Harry Dunn are "very low".
Anne Sacoolas, 42, left the UK shortly after the incident outside RAF Croughton in Northamptonshire on August 27 and subsequently claimed diplomatic immunity.
Sacoolas was charged with causing the death of 19-year-old Harry by dangerous driving.
She was twice interviewed by Northamptonshire Police - once on the day after the crash, and on another occasion by officers who travelled to the US.
Before Christmas the force charged the US diplomat's wife with causing death by dangerous driving.
Last week the Home Secretary confirmed that the government had made an extradition request.
But the US State Department described it as "highly inappropriate" and Sacoolas's legal team have indicated she is not willing to return.
Today the Prime Minister told BBC Breakfast: "I think that it's right that we made the appeal for extradition."
But he added: "I think the chances of America actually responding by sending Anne Sacoolas to this country are very low.
That's not what they do."
Harry's parents Tim Dunn and Charlotte Charles have urged Mrs Sacoolas to return to the UK to "face justice".
Reacting to the Prime Minister's comments from Denver in the US, the Dunn family spokesman Radd Seiger said he believed that the US diplomat's wife would return.
He said the family were ready to begin a campaign, which would include blockading the RAF base, in order to pressure Sacoolas to return.
He said: "I do not know what is in the Prime Minister's mind in making those comments because the parents and I have not yet had the opportunity to sit down and talk with him - but we expect to do so within the next few days.
"Certainly, if he is basing those comments on what is currently emanating from Washington he may well be right.
"Harry's family will as always take things one step at a time and will not prejudge the outcome of the process but as I have stated many times publicly, my analysis of the prospects of success are diametrically opposed to Mr Johnson's given my detailed discussions with officials both in London and Washington.
"If that is what transpires, the campaign will swing into action deploying a number of measures including blockading the bases and will sit down with British officials to discuss what they will do, not only on Harry's family's behalf, but the whole nation's to ensure that justice is done and that no-one ever suffers the same fate as Harry.
"That is not a sustainable position and I have made that crystal clear to the Government and the point has been taken on board.
"But we will take one step at a time and not get ahead of ourselves. Anne Sacoolas will be coming back to the UK to face justice. Unlike the Prime Minister, there is no doubt in my mind."
