Discussions relating to the case of British-Iranian mother Nazanin Zaghari-Ratcliffe, who is being held in Iran, have been described as “delicate” by Boris Johnson who said he did not want to “tempt fate” regarding any progress being made.

Hopes had been raised that the she might finally be released, after an MP said on Tuesday that the 43-year-old’s British passport had been returned to her.

Hampstead and Kilburn MP Tulip Siddiq also said a British negotiating team is in Tehran, while Mrs Zaghari-Ratcliffe remains at her family home in the Iranian capital.

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

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.

She has always denied the allegations against her.

Her MP Ms Siddiq tweeted: “I am very pleased to say that Nazanin Zaghari-Ratcliffe has been given her British passport back. She is still at her family home in Tehran. I also understand that there is a British negotiating team in Tehran right now. I will keep posting updates as I get them.”

Asked about the case, the Prime Minister said: “I don’t want to tempt fate”.

“Negotiations about all our difficult consular cases have been going on for a long time,” he told reporters, saying “quite delicate discussions” are still taking place.

A £400 million debt relating to a cancelled order for 1,500 Chieftain tanks dating back to the 1970s had been linked to the continued detention of Mrs Zaghari-Ratcliffe and other UK-Iranian dual nationals held in the country.

In December, Foreign Secretary Liz Truss said the £400 million that Britain owes Iran is a “legitimate debt” that the Government wants to pay.

On Tuesday, a No 10 official said of the debt: “There is no change on our position to that.

“We are committed to paying the debt. We’re exploring options to resolve it – it has not been resolved.”

Pressed on whether a UK negotiating team is in Tehran, he added: “I’m not going to get into further speculation at this point.”

Mr Johnson was also reluctant to say too much, 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.

“Everybody wants Nazanin home, we have been working on that for a long, long time.

“I do not want to do anything to interrupt the conversations right now.”

According to her employer, the Thomson Reuters Foundation, Mrs Zaghari-Ratcliffe’s lawyer Hojjat Kermani, when asked whether she will be released, said: “I am hopeful that we will have good news soon.”

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

Mrs Zaghari-Ratcliffe’s mother-in-law, Barbara Ratcliffe, told the BBC the latest news “does sound really positive” and that her daughter-in-law had “seemed really quite upbeat when I last spoke to her”.

But she added that the family is “all a bit battle-scarred”, having had so many disappointments in the past regarding progress on the case.

Mrs Zaghari-Ratcliffe’s sister-in-law Rebecca Ratcliffe echoed those sentiments.

Of the passport being returned, she told BBC London: “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.”

London mayor Sadiq Khan welcomed the “positive news” about the passport but added that Mrs Zaghari-Ratcliffe “must now be reunited with her family in the UK as soon as possible” and said “London continues to stand with her as we all fervently hope for her return to our city”.

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.

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”.

Human rights organisations Amnesty International UK and Redress urged caution around the latest update, saying there had been “false dawns” previously.

In relation to the debt, the Foreign, Commonwealth and Development Office said: “We continue to explore options to resolve this case and will not comment further as discussions are ongoing.”

On the cases of Britons detained, a Foreign Office spokesperson said: “We have long called for the release of unfairly detained British nationals in Iran. We don’t comment on speculation.”