Boris Johnson’s assertion that the wife of a US intelligence officer charged with causing the death by dangerous driving of the 19-year-old motorcyclist Harry Dunn is unlikely to be sent back to the UK to face trial is misconceived, a spokesman for the teenager’s family has said.

The Home Office confirmed on Friday that it had sent a request to the US government to extradite Anne Sacoolas over the collision outside RAF Croughton, in Northamptonshire, on 27 August last year.

However, on Tuesday, in an interview with BBC One’s Breakfast, the prime minister said the chances of the US sending her to the UK were “very low”, adding: “That’s not what they do.”

Responding to Johnson’s comments, the spokesman for the Dunn family, Radd Seiger, said that even if an extradition request was blocked, it would not stop Sacoolas facing justice.

“That is not a sustainable position and I have made that crystal clear to the government and the point has been taken onboard,” he told PA Media. “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.”

Seiger, who is in Denver, said he and Dunn’s parents hoped to meet Johnson in the coming days to discuss the case. In response to the extradition request, the US state department described it as “highly inappropriate”, claiming that because Sacoolas was the spouse of a former diplomat, it would establish “an extraordinarily troubling precedent”.

Seiger said that if Johnson was basing his comments on statements coming out of Washington then he was probably right. But he added: “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.

“Should the US secretary of state, Mike Pompeo, ultimately make the political decision not to return her, that will not be the end of the matter.”

He said that if that happened a campaign would swing into place, including a blockade of US military bases in the UK. Earlier this month, almost 100 friends and supporters of the Dunn family temporarily blocked the entrance to RAF Croughton, demanding that US authorities comply with the UK extradition process.

On Breakfast, Johnson said: “I think that it’s right that we made the appeal for extradition.” 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.”

Seiger tweeted that the prime minister’s comments were not backed by precedent.

I am afraid the PM is misinformed. The US has never once, in the long history of the treaty, turned down an extradition request from the UK. Not once. https://t.co/WzVuyCnEjz — Radd Seiger (@RaddSeiger) January 14, 2020

Dunn died after his motorbike collided with a car outside the RAF base. Sacoolas, 42, claimed diplomatic immunity after the incident and was able to return to her home country, prompting an outcry in the UK and tensions with the US. Her lawyer has said she will not return to the UK “to face a potential jail sentence for what was a terrible but unintentional accident”.

Extradition requests go through the US State Department, Department of Justice and US courts, but the ultimate decision lies with the secretary of state.