Boris Johnson has said the UK was doing “everything we can” in the interests of Nazanin Zaghari-Ratcliffe, as officials downplayed reports from Iran that said Britain would pay a £400 million debt to secure her release.

Iranian state TV claimed on Sunday that a deal had been struck over the long-running dispute, which has been suggested as a reason for the British-Iranian charity worker’s detention.

But UK officials have since downplayed the idea that payment of the debt would mean Mrs Zaghari-Ratcliffe’s imminent release, while the Prime Minister said on Monday they were “two entirely separate issues”.

“We of course make sure that we do everything we can to look after the interests of Nazanin and all the very difficult dual national cases we have in Tehran,” Mr Johnson told reporters during a campaign visit to Hartlepool.

The mother-of-one’s husband Richard Ratcliffe told the PA news agency on Sunday the family had not been updated, but welcomed the signals from Tehran as “a good sign”.

Meanwhile, Foreign Secretary Dominic Raab said reports that Britain would pay the debt were “not yet accurate”.

He told a Downing Street press conference on Monday: “It’s incumbent on Iran unconditionally to release those who are held arbitrarily and, in our view, unlawfully, and the reports I’m afraid are not yet accurate in terms of the suggestion of her (Ms Zaghari-Ratcliffe’s) imminent release.”

The legal dispute dates back to the 1970s when the then-shah of Iran paid the UK £400 million for 1,500 Chieftain tanks.

Britain refused to deliver the tanks to the new Islamic Republic when the shah was toppled in 1979, but kept the cash despite British courts accepting it should be repaid.

The Foreign Office said “legal discussions are ongoing” over the debt despite the claim made on Iranian state TV, which cited an anonymous official.

Foreign Office minister James Cleverly said ongoing legal negotiations between Tehran and London should be kept separate from the “arbitrary detention” of prisoners there.

“We have always said that British dual nationals should not be used as political leverage,” Mr Cleverly told Sky News.

“We have also seen a number of occasions where the Iranian regime have used disinformation, we’re hearing inaccurate reports coming out over the last couple of days.

“On the one hand, they are saying that these proceedings are legitimate. We don’t agree with that at all, but then also saying that they are linked to this legal dispute – it can’t be both.

“We’re making it very, very clear. It is in the hands of the Tehran regime to release these people and they should be released.”

The anonymous official was also quoted saying a deal had been made between the US and Tehran for a prisoner swap in exchange for the release of seven billion dollars (£5 billion) of frozen Iranian funds.

But Washington denied the report, saying suggestions of a prisoner swap were “not true”.

Mrs Zaghari-Ratcliffe, 42, of north London, was detained in Tehran in 2016 while taking daughter Gabriella to see her family, as authorities made widely refuted allegations of spying.

She completed a five-year sentence in March, having carried out hunger strikes in protest over her treatment in jail as diplomatic efforts were made to secure her freedom.

But she and her family were delivered a fresh blow last week when she was given an additional one-year jail term, while she was also banned from leaving Iran for a further year.