Boris Johnson has said negotiations with Iran to free Nazanin Zaghari-Ratcliffe are “going right up to the wire”.

       The prime minister raised hopes that the dual national’s six-year ordeal could come to a close after suggestions on Tuesday that the mother of one has had her passport returned.

       But Mr Johnson, during a trip to the Middle East, was cautious not to elaborate further on the state of negotiations with Tehran because talks are ongoing.

       A glimmer of optimism for the 43-year-old came a day earlier when her constituency MP in Hampstead and Kilburn, Tulip Siddiq, said Mrs Zaghari-Ratcliffe had been returned her British passport.

       A negotiating team from the UK is currently working in Tehran to secure the release of dual nationals, while Mrs Zaghari-Ratcliffe remains at her family home in the Iranian capital.

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       Speaking on Wednesday to broadcasters at the Emirates Palace hotel in Abu Dhabi, Mr Johnson said: “I shouldn’t really say much more right now just because those negotiations continue to be under way and we’re going right up to the wire.”

       Mrs Zaghari-Ratcliffe was arrested in April 2016 as she prepared to fly back to the UK, having taken her daughter Gabriella – then not even two years old – to see relatives.

       Nazanin Zaghari-Ratcliffe was arrested in April 2016

       (PA)

       She was accused of plotting to overthrow the Iranian government and sentenced to five years in jail, spending four years in Tehran’s Evin Prison and one under house arrest.

       Both the UK government and Mrs Zaghari-Ratcliffe have always denied the allegations.

       While the details of the negotiations remain unclear, it is possible they are linked to a ￡400 million debt dating back to the 1970s owned to Iran by the UK.

       The government accepts it should pay the “legitimate debt” for an order of 1,500 Chieftain tanks that was not fulfilled after the shah was deposed and replace by a revolutionary regime.

       Nazanin Zaghari-Ratcliffe – in pictures Show all 20

       1/20Nazanin Zaghari-Ratcliffe – in pictures

       Nazanin Zaghari-Ratcliffe – in pictures 2018 Nazanin Zaghari-Ratcliffe hugs her daughter Gabriella, in Iran after she was allowed to leave the Iranian prison, she is being held in, for three days. Zaghari-Ratcliffe was arrested during a holiday with her toddler daughter in April 2016. Iranian authorities accuse her of plotting against the government. Her family denies this, saying says she was in Iran to visit family.

       Free Nazanin Campaign/AP

       Nazanin Zaghari-Ratcliffe – in pictures Nazanin Zaghari-Ratcliffe with her husband Richard Ratcliffe and their daughter Gabriella. Nazanin is serving a five-year prison sentence for allegedly plotting to overthrow Iran's government.

       PA

       Nazanin Zaghari-Ratcliffe – in pictures June 2016 Richard Ratcliffe's daughter Gabriella had her British passport confiscated and was stranded in Iran with her grandparents after her mother Nazanin was jailed. He left left a giant birthday card on the doorstep of the Iranian embassy in central London to mark her second birthday in June 2016.

       PA

       Nazanin Zaghari-Ratcliffe – in pictures Nazanin has spent some of her prison sentence in solitary confinement.

       PA

       Nazanin Zaghari-Ratcliffe – in pictures Nazanin Zaghari-Ratcliffe with her husband Richard and daughter Gabriella.

       Family Handout

       Nazanin Zaghari-Ratcliffe – in pictures July 2016 Richard Ratcliffe delivering a letter of petition with his mother Barbara Ratcliffe and MP Tulip Siddiq, to 10, Downing Street on the 100th day of her detention, on July 12, 2016.

       Getty

       Nazanin Zaghari-Ratcliffe – in pictures Supporters of Nazanin Zaghari-Ratcliffe held a vigil outside the Foreign and Commonwealth Office to mark her 707 days in captivity.

       Getty

       Nazanin Zaghari-Ratcliffe – in pictures January 2017 Richard Ratcliffe holds a '#Free Nazanin' sign and candle during a vigil for for wife on January 16, 2017. The vigil, being held outside the Iranian Embassy in London marks one year since the Washington Post journalist Jason Rezaian and other US-Iranian dual-nationals were released from prison in Iran.

       Getty

       Nazanin Zaghari-Ratcliffe – in pictures Nazanin with her daughter Gabriella before they were detained by Iranian authorities.

       Change.org

       Nazanin Zaghari-Ratcliffe – in pictures September 2017 Gabriella, who is three-years-old in this picture, has now spent two years away from her mother.

       Richard Ratcliffe

       Nazanin Zaghari-Ratcliffe – in pictures November 2017 Foreign Secretary Boris Johnson meets with Richard Ratcliffe over Nazanin's case. They meet just days after Johnson told a parliamentary committee that she was in Iran "training journalists".

       WPA Pool/Getty

       Nazanin Zaghari-Ratcliffe – in pictures November 2017 Actor Emma Thompson braved pneumonia to support Richard Ratcliffe in leading demonstrators before a march in support of Nazanin in November.

       Reuters

       Nazanin Zaghari-Ratcliffe – in pictures November 2017 Richard Ratcliffe after the march said: 'It is profoundly moving to see so many people here.'

       REUTERS

       Nazanin Zaghari-Ratcliffe – in pictures November 2017 A picture of Nazanin Zaghari-Ratcliffe shown on Iranian state TV as part of a report that made fresh allegations against her. They said she had been recruiting for banned broadcast services, as well as 'opposition cyber teams'.

       Nazanin Zaghari-Ratcliffe – in pictures December 2017 Iranian president Hassan Rouhani greets British Foreign Secretary Boris Johnson at the presidential office in Tehran, Iran. Johnson visited Tehran to discuss the fate of detained Nazanin Zaghari-Ratcliffe.

       EPA

       Nazanin Zaghari-Ratcliffe – in pictures Nazanin Zaghari-Ratcliffe with her daughter Gabriella.

       PA

       Nazanin Zaghari-Ratcliffe – in pictures December 2017 Photos of Richard Ratcliffe and his wife Nazanin Zaghari-Ratcliffe on display at their home in north London. Mr Ratcliffe said he believed there was "still a chance" she may be released from an Iranian prison in time for a dream Christmas together. Unfortunately that didn't happen.

       PA

       Nazanin Zaghari-Ratcliffe – in pictures February 2018 Richard Ratcliffe delivers a petition and a letter addressed to the Iranian Deputy Foreign Minister Abbas Araghchi to demand her release, at the Iranian Embassy in London on February 21, 2018. He also left support letters for his spouse in the country's embassy, amid a visit by a deputy foreign minister.

       AFP/Getty

       Nazanin Zaghari-Ratcliffe – in pictures August 2018 Foreign Secretary, Jeremy Hunt meeting Richard Ratcliffe. Hunt has pledged to do everything possible to secure the release of a charity worker jailed in Iran

       Jeremy Hunt/PA

       Nazanin Zaghari-Ratcliffe – in pictures August 2018 Nazanin Zaghari-Ratcliffe hugs her daughter Gabriella, in Iran after she was allowed to leave the Iranian prison, she is being held in, for three days. Zaghari-Ratcliffe was arrested during a holiday with her toddler daughter in April 2016. Iranian authorities accuse her of plotting against the government. Her family denies this, saying says she was in Iran to visit family.

       PA

       Tehran remains under strict sanctions, however, which have been linked to the failure to clear the debt.

       Foreign secretary Liz Truss on Wednesday said the UK was working “very hard” to secure Mrs Zaghari-Ratcliffe’s release, adding it was also a “priority to pay the debt that we owe to Iran” .

       Meanwhile, Mrs Zaghari-Ratcliffe’s sister-in-law Rebecca Ratcliffe said the family are trying not to raise their hopes too much after the latest developments, having been left disappointed in the past.

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       She told BBC Breakfast: “We know that there’s a delegation, a UK delegation, gone over there to talk to Iranian officials and hopefully negotiate so it seems like a very positive step in Nazanin’s case, and hopefully coming close to an end but we just don’t know how far it’s gone.”

       Ms Ratcliffe said the case could come to a close very quickly or could be “a stunt” on Iran’s part.

       She said: “This may all be over in a week or two. But also it may just be a stunt from the Iranians. We’ve had this before. You know, we’ve had many ups over the last six years and been told she’s been about to be released. So there’s an element of false hopes and I think our family, Nazanin, her parents, find it hard to get too excited at the moment.”