Nazanin Zaghari-Ratcliffe was detained at an Iranian airport in April 2016, while travelling home to London with her young daughter.
The British-Iranian was sentenced to five years in prison - for allegedly plotting against the Iranian government.
She maintains her innocence, saying she was on holiday in Iran visiting family.
Who is Nazanin Zaghari-Ratcliffe?
Mrs Zaghari-Ratcliffe, 41, has dual British and Iranian citizenship, and before her arrest lived in London with her husband Richard Ratcliffe, who is an accountant.
She worked as a project manager for the charity Thomson Reuters Foundation and was previously employed by BBC Media Action, an international development charity.
She says she took her daughter Gabriella, now six, to Iran in March 2016 to celebrate the country's new year and visit her parents.
What is she accused of?
Iranian authorities allege Mrs Zaghari-Ratcliffe was plotting to topple the government in Tehran - but no official charges have been made public.
Iran's Revolutionary Guards said she was leading a "foreign-linked hostile network" when she visited.
Both the Thomson Reuters Foundation and BBC Media Action issued statements saying she was not working in Iran but was on holiday there.
Mr Ratcliffe said he had asked his wife what the charges were during a phone call in 2016, and a nearby prison guard had said: "National security charges".
What has the UK government done?
In October last year, Foreign Secretary Dominic Raab said the government would "redouble" its efforts to secure Mrs Zaghari-Ratcliffe's release after her daughter Gabriella returned to the UK.
A month earlier, newly-appointed prime minister Boris Johnson had called for Mrs Zaghari-Ratcliffe's immediate release during a meeting with Iran President Hassan Rouhani.
He had previously been involved in the case when he was foreign secretary. In November 2017, he had to apologise after he was criticised for suggesting Mrs Zaghari-Ratcliffe had been training journalists when she was arrested, rather than on holiday.
Campaigners said she could face an increased prison sentence in Iran as a result of the comments.
The following month Mr Johnson visited Tehran and met his counterpart as well as President Rouhani, and pressed for her release on humanitarian grounds.
Theresa May had also raised concerns when she was prime minister.
In March 2019, then-Foreign Secretary Jeremy Hunt - Mr Johnson's successor - announced Mrs Zaghari-Ratcliffe would be given diplomatic protection - making the case a formal, legal dispute between Britain and Iran.
What happened after her arrest?
- April-June 2016: Mr Ratcliffe said his wife was subject to "intense interrogation" for the first two months of her imprisonment and kept in solitary confinement
- September 2016: Mrs Zaghari-Ratcliffe was sentenced to five years in prison by Tehran's Revolutionary Court
- April 2017: Mrs Zaghari-Ratcliffe lost a final appeal in Iran's supreme court to overturn her sentence
- August 2018: She was reunited with her daughter while on a temporary three-day release
- January 2019: Mr Ratcliffe said his wife started a three-day hunger strike to protest about a lack of specialist medical treatment
- June 2019: Mrs Zaghari-Ratcliffe undertook a 15-day hunger strike, calling for her unconditional release
- October 2019: Mrs Zaghari-Ratcliffe's daughter Gabriella returned to the UK to start school
- March 2020: Mrs Zaghari-Ratcliffe was given temporary leave from prison because of the pandemic and has been living at her parents' house in Tehran
- September 2020: She was told she would face another trial over a new charge
What has Mrs Zaghari-Ratcliffe said?
Mrs Zaghari-Ratcliffe, who maintains her innocence, mostly speaks publicly through her husband, who regularly calls her.
According to Mr Ratcliffe, she misses Gabriella "all the time" and is in a "pretty fragile" state of mental health.
In an open letter, published in October 2019, Mrs Zaghari-Ratcliffe said weekly visits from her daughter had sustained her during her imprisonment.
Writing before Gabriella returned to the UK to start school, she said the thought of being unable to hold her child was "the deepest torture of them all".
What are the politics behind her case?
Mrs Zaghari-Ratcliffe's five-year-sentence was handed down on 6 September - one day after the UK appointed an ambassador to Iran for the first time since 2011.
Mr Ratcliffe said the timing was proof his wife is being held as a political bargaining chip.
It has been claimed Mrs Zaghari-Ratcliffe is being held in order to force the UK into settling a multi-million pound dispute with Iran, which dates back to the 1970s.
BBC diplomatic correspondent Caroline Hawley said London and Tehran do not see eye-to-eye on Iran's role in the wider Middle East or on human rights.
Iran's Revolutionary Guards are hardliners, opposed to Iran's nuclear deal struck by moderates with the West in 2015. And in 2009, Britain was accused of fuelling unprecedented street protests.
Tensions between the nations increased last year after a series of incidents in and around the Gulf, including Iran seizing a British-flagged tanker in the Strait of Hormuz.