The refusal to grant an extradition request for the suspect charged in connection with the death of Harry Dunn is a “denial of justice”, the prime minister has said.

Boris Johnson’s comments were welcomed by the teenager’s mother, Charlotte Charles, who said they were “words that need action behind them” on Monday.

Johnson’s criticism of the US secretary of state’s decision to reject the request for the return of Anne Sacoolas to the UK, follows similar comments from the foreign secretary, Dominic Raab, and the home secretary, Priti Patel.

On Monday, Dunn’s parents met with Raab, describing the talks as “very honest” and “positive”.

Charles said she could see Raab’s “anger” with Mike Pompeo’s decision to reject the extradition request for Sacoolas.

Speaking outside the Foreign Office after the meeting, the family spokesman Radd Seiger said: “We wanted to look at the foreign secretary’s eyes and make it absolutely clear we were not going to accept Harry being swept under the carpet.

“The parents and I are absolutely clear that that is not the case and the foreign secretary is every bit as angry as the parents are, and frankly the whole nation now and they are not going to take this lying down.”

Dunn was killed when his motorbike crashed into a car outside a US military base in Northamptonshire on 27 August last year.

Sacoolas, the 42-year-old wife of a US intelligence official based at RAF Croughton, was granted diplomatic immunity after the crash, sparking an international controversy.

Pompeo rejected an extradition request by the UK government for Sacoolas on Thursday.

On Monday, the prime minister’s spokesman said: “We believe that this is a denial of justice and the individual concerned should return to the UK.”

Johnson also “reiterated the need” for Sacoolas to return to the UK during a phone call with Donald Trump on Friday.

Reacting to Johnson’s comments, Charles said the prime minister needed to put pressure on Trump and Pompeo to send the suspect back to the UK.

“The fact that he has come out and said that is obviously a good thing but we hope that he can follow this up with actions now. He needs to use his power, because he’s the only person with the power to tell Trump and Pompeo that she needs to come back. It feels good that he’s on the same page as us and this is certainly a step in the right direction,” she said.

Asked what her message to the prime minister would be, Charles added: “Make the country proud of you. There are a lot of people out there who don’t necessarily have faith in this government and this is a chance for him to restore a bit of that faith.

“It’s certainly another step forward but, until I see action, they are not empty words, but they are words that need action behind them to show us that they are 100% going to stand up for us.”

Addressing her views on the meeting with Raab, Charles said: “Dialogue was very open, everything was very honest, every question we had was answered. He was able to look us in the eye, we could see his anger with the decision of the States turning down the extradition request.

“We didn’t realise how large this would become or how much work it would take but we don’t regret a day of it.”

Charles and Dunn’s father, Tim Dunn, have disputed the Foreign Office’s claim that the suspect had diplomatic immunity at the time of the road crash. Pompeo is due to meet Raab on Wednesday.