Talks over the case of Nazanin Zaghari-Ratcliffe are at a “delicate” stage, Boris Johnson said on Tuesday after the detained British-Iranian mother had her passport returned by Iranian authorities.

       The move had sparked hopes that Ms Zaghari-Ratcliffe might finally be released – but the prime minister declined to comment in detail, saying he did not want to “tempt fate.”

       Her lawyer, Hojjat Kermani, told Reuters: “I am hopeful that we will have good news soon.”

       Ms Zaghari-Ratcliffe has been detained in Iran since 2016, when she was arrested on charges of plotting to overthrow the government.

       Her ongoing imprisonment, and that of other UK-Iranian dual nationals held in the country, has been linked to a ￡400m debt relating to a cancelled order for 1,500 Chieftain tanks dating back to the 1970s.

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       Iran’s clerical rulers say Britain owes the money that Iran’s Shah paid up front.

       Ms Zaghari-Ratcliffe’s MP, Tulip Siddiq, said her constituent was still at her family home in Tehran and that she understands a British negotiating team is in the capital.

       “I am very pleased to say that Nazanin Zaghari-Ratcliffe has been given her British passport back,” Ms Siddiq said.

       “I will keep posting updates as I get them,” the Labour MP for Hampstead and Kilburn added.

       Speaking to the BBC, Rebecca Ratcliffe, Ms Zaghari-Ratcliffe’s sister-in-law, said: “It does seem like some kind of recognition that the Iranians are getting ready for her to be allowed home at some point.

       “I think she’s so tired of reading too much into things and until she’s on that flight home she just can’t believe that release is imminent so it is a big step forward, we just don’t quite know how to interpret it at the moment.”

       In March, Ms Zaghari-Ratcliffe was released from house arrest and had her ankle tag removed after serving a five-year jail term, but she was rearrested in April by the Iranian regime on propaganda charges.

       Mr Johnson's official spokesperson said the debt to Iran has not been repaid, and restated Britain’s official position that there is no link between the money and Ms Zaghari-Ratcliffe’s case.

       Zaghari-Ratcliffe with her daughter Gabriella in 2018, while Gabriella was living with her grandparents in Tehran

       (PA Media)

       “There is no change to our position on that,” said the spokesperson. “We are committed to paying the debt. We are exploring options to resolve it. It has not been resolved.”

       He added that discussions were “ongoing” in relation to other dual nationals being held by Iran, saying: “We have long called for the release of unfairly detained British nationals in Iran.”

       Mr Johnson was also reluctant to elaborate, telling reporters: “I think that it’s very important when you have got quite delicate discussions going on, negotiations going on in Tehran about some of our most difficult consular cases – particularly Nazanin Zaghari-Ratcliffe – you should say as little as possible unless and until the thing is actually concluded.”

       Ms Zaghari-Ratcliffe’s husband, Richard Ratcliffe, spent 21 days on hunger strike last year in London to draw attention to his wife’s case.

       In January, the daughter of another British-Iranian detained in Iran said her father was to begin a hunger strike due to a lack of progress in securing his release.

       Retired civil engineer Anoosheh Ashoori has been held at Evin Prison on charges of spying for Israel, which he denies, for more than four years.

       Richard Ratcliffe

       (PA)

       In a video posted in relation to the hunger strike, Elika Ashoori said she was at that time “extremely concerned” for her father’s health “as he approaches his 68th birthday”.

       In December, foreign secretary Liz Truss said the ￡400m that Britain owes Iran is a “legitimate debt” that the government wants to pay.

       Sacha Deshmukh, Amnesty International UK’s chief executive, warned that the latest reports should be treated with caution as there had been “false dawn after false dawn” in the long-running process.

       He said: “We sincerely hope these reports are correct.

       “The detainees and their families have been suffering for years, and a resolution can’t come quickly enough.

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       “It’s been clear for a long time that the Iranian authorities have been targeting foreign nationals with spurious national security-related charges to exert diplomatic pressure.

       “In the past we’ve had false dawn after false dawn over possible breakthroughs, so it’s only right to be cautious at the moment.”