Nazanin Zaghari-Ratcliffe's family is staging a protest outside the Iranian embassy after she was summoned back to a court in the country on fresh charges of propaganda activities.
Mrs Zaghari-Ratcliffe has now completed her near five-year prison sentence, over widely refuted allegations of plotting to overthrow Iran's government.
She strongly denies the charges.
But after having her ankle tag removed, having finished the latter part of her sentence under house arrest due to Covid-19, an Iranian court has set a date for her to return to face new charges.
The Guardian reports the new allegations she faces – long threatened by Iranian authorities – concern Mrs Zaghari-Ratcliffe’s involvement in propaganda activity against Iran, including attending a 2009 demonstration outside its embassy in London, and speaking to BBC Persian.
Richard Ratcliffe, the couple's daughter, and Ms Zaghari-Ratcliffe's brother will demonstrate outside Iran's embassy in Knightsbridge on Monday.
Mr Ratcliffe will deliver a 60,000-signature Amnesty International petition to the embassy calling for his wife’s immediate release.
Mr Ratcliffe called it a “watershed” moment, saying: "If you’d asked me when we first started campaigning with Amnesty to bring Nazanin home that five years later we’d still be knocking on the door of the Iranian embassy, still waiting for them to ever open it and explain what’s going on, then I would have been horrified."
He earlier said he was "grateful" for the "strong words" of Foreign Secretary Dominic Raab, who condemned the "cruel and intolerable" treatment of Mrs Zaghari-Ratcliffe and called for her swift return to the UK.
Prime Minister Boris Johnson - who faced criticism for his handling of the case when he was foreign secretary - has urged Iran to release her "permanently", adding her "continued confinement remains totally unacceptable".
Mrs Zaghari-Ratcliffe, of north London, was arrested at Tehran’s Imam Khomeini Airport while taking daughter Gabriella to see her parents in April 2016.
The charity worker, who was employed by the Thomson Reuters Foundation at the time of her arrest, strongly denies the charges and civil rights groups say she was jailed with no evidence and her trial was unfair.
The UK has been locked in a high-profile diplomatic tussle over Mrs Zaghari-Ratcliffe’s detention, which has seen her held in solitary confinement and undergo hunger strikes.
Rupert Skilbeck, director of the Redress legal campaigning group, said the “cumulative effect” of her confinement over the past half decade “crosses that threshold into torture” and warned over long-term psychological effects.
The UK government has afforded her diplomatic protection, arguing she is innocent and that her treatment by Iran failed to meet obligations under international law.
Mrs Zaghari-Ratcliffe has been used as a political pawn, according to Nobel Laureate and Iranian human rights lawyer Shirin Ebadi.
Commentators have linked a long-standing debt running into hundreds of millions of pounds as central to the case, which has been dubbed "hostage diplomacy" by former foreign secretary Jeremy Hunt.
The UK is thought to owe Iran as much as £400 million over the non-delivery of tanks in 1979, with the shipment stopped because of the Islamic revolution.
Mrs Zaghari-Ratcliffe’s constituency MP Tulip Siddiq (Hampstead and Kilburn) has connected the debt with the case.
She said last month: "My constituent’s life is basically a bargaining chip because she’s not being set free because we haven’t fulfilled our responsibility of paying the debt.
"If there’s some movement on that I reckon Nazanin’s chances are increased."