Zaghari-Ratcliffe was released from house arrest in the country and had her ankle tag removed at the weekend after completing her five-year prison sentence.
But the dual national has been unable to return to the UK and is scheduled to appear in court to face a second set of charges in March, according to her lawyer.
So, who is Nazanin Zaghari-Ratcliffe, why was she jailed in Iran, what is Mr Johnson’s involvement in the case - and will she return to the UK?
Here is everything you need to know.
Who is Nazanin Zaghari-Ratcliffe?
Before her arrest, Nazanin Zaghari-Ratcliffe lived in London with her husband Richard Ratcliffe, an accountant, and her daughter Gabriella.
The 41-year-old, who has dual British and Iranian citizenship, worked as a project manager for charity Thomson Reuters Foundation and was previously employed by international development charity BBC Media Action.
Zaghari-Ratcliffe took her daughter, now six, to visit Iran in March 2016.
She says the trip was to celebrate the new year in the country and to visit her parents.
But the British-Iranian was detained at an Iranian airport in April 2016 when travelling home to London with Gabriella.
Why was she jailed in Iran?
Following her arrest, Zaghari-Ratcliffe was sentenced to five years in prison for allegedly plotting to overthrow the Iranian government.
Iran’s Revolutionary Guards has said the dual national was leading a "foreign-linked hostile network" when she visited the country.
She maintains her innocence, saying she was on holiday in Iran visiting her family, and mostly speaks publicly through her husband who regularly calls her.
Ratcliffe said he had asked his wife what the charges were during a 2016 phone call, when a nearby prison guard said: "National security charges".
The Thomson Reuters Foundation and BBC Media Action both issued statements clarifying that Zaghari-Ratcliffe was not working in Iran but on holiday.
Zaghari-Ratcliffe’s sentence included eight months of solitary confinement and blindfolded interrogations.
She also went on two hunger strikes to protest at her conditions.
Zaghari-Ratcliffe has been denied consular access since Iran does not recognise her dual citizenship.
The UK Government has granted her diplomatic protection, meaning the case is treated as a formal, legal dispute between the two countries.
Ratcliffe believes that his wife, among other dual nationals, is being held hostage as Iran wants Britain to pay more than £400million in debt which dates back to a 1970s arms deal.
Yet the UK has said it can’t pay the sum due to sanctions against Iran.
Why was Nazanin Zaghari-Ratcliffe moved from prison?
Zaghari-Ratcliffe was moved from jail in March 2020 due to the coronavirus pandemic.
She was put under house arrest at her parents’ home in Tehran.
When her five-year sentence ended, Zaghari-Ratcliffe’s ankle tag - which stopped her from going more than 300 metres from the house - was removed.
Her lawyer said: “She was pardoned by Iran’s supreme leader last year, but spent the last year of her term under house arrest with electronic shackles tied to her feet. Now they’re cast off. She has been freed.”
Speaking after the ankle bracelet was removed on Sunday 7 March, Ratcliffe told the PA news agency that Zaghari-Ratcliffe was “genuinely happy” to be free of the tag.
But he added: "I'm a bit more guarded - it feels to me like they have made one blockage just as they have removed another, and we very clearly remain in the middle of this government game of chess.”
What are the new charges?
The dual national will have to go to court to face a new set of charges on 14 March, her lawyer said.
The fresh charges include involvement in propaganda activity against the Islamic Republic by attending a protest outside the Iranian embassy in London in 2009, as well as talking to BBC Persian - a network which Tehran believes spreads disinformation.
The evidence which may make up the basis of a new prosecution case has already been made public by Iranian officials, and was available at Zaghari-Ratcliffe’s first trial.
Iranian authorities had warned about the new set of charges four years ago and on following occasions, including in September 2020, but had held back from carrying through with it.
What did Boris Johnson say about her case?
Boris Johnson has pledged to do what he can to secure the British-Iranian’s permanent release.
In a phone call with Hassan Rouhani on Wednesday 10 March, the Prime Minister told Iran’s president that the treatment of Zaghari-Ratcliffe was “completely unacceptable”.
He demanded the former charity worker’s “immediate release”, along with that of other British-Iranians detained in the country.
A Downing Street spokesperson said: "He said that while the removal of Nazanin Zaghari-Ratcliffe's ankle monitor was welcome, her continued confinement remains completely unacceptable and she must be allowed to return to her family in the UK.”
And writing on Twitter on 7 March, Mr Johnson said: “Pleased to see the removal of Nazanin Zaghari-Ratcliffe’s ankle tag, but her continued confinement remains totally unacceptable.
“She must be released permanently so she can return to her family in the UK, and we continue to do all we can to achieve this.”
But Zaghari-Ratcliffe’s husband has previously criticised Mr Johnson for his involvement in the case.
When he was foreign secretary, Mr Johnson wrongly told a House of Commons committee that Zaghari-Ratcliffe was “teaching people journalism” in Iran during her visit.
Iranian officials then cited the UK politician’s words as evidence that Zaghari-Ratcliffe had engaged in “propaganda against the regime”.
Will Zaghari-Ratcliffe return to the UK?
As Zaghari-Ratcliffe is facing a second set of charges, it is unlikely that she will be given back her passport which she would need to come back to Britain.
Dominic Raab, UK foreign secretary, said of the dual national’s situation: “She must be allowed to return to the UK as soon as possible to be reunited with her family.
“We will continue to do all we can to achieve this. We have relayed to the Iranian authorities in the strongest possible terms that her continued confinement is unacceptable.”
His predecessor Jeremy Hunt added that it was “beyond cruel to toy with an innocent mother and six-year-old child in this way”, saying Iran’s “hostage diplomacy must stop”.