Boris Johnson has raised Nazanin Zaghari-Ratcliffe’s “completely unacceptable” case directly with Iranian President Hassan Rouhani.
Downing Street said the two leaders spoke on Wednesday afternoon and the Prime Minister demanded her immediate release, along with other detained British-Iranian dual nationals.
A No 10 spokesman 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.”
Ms Zaghari-Ratcliffe was released from house arrest and had her ankle tag removed at the weekend after a five-year prison term expired.
She was detained in 2016 as Iranian authorities made widely refuted spying allegations, and finished the latter part of her sentence under house arrest due to the Covid-19 pandemic.
But she could face further charges and has been unable to return to the UK.
Her husband Richard Ratcliffe, accompanied by the couple’s six-year-old daughter Gabriella, attended a vigil outside the Iranian embassy in Knightsbridge, central London, on Monday.
Speaking to Times Radio, Mr Ratcliffe said it will be difficult for his wife to see her family in Iran again once she can return to the UK.
“That is part of the hardship, perhaps in the early period of realising that actually when they took her into prison, they took away Iran from her forever,” he told host Phil Williams.
“I don’t think she’s going around seeing everyone thinking ‘this is the last time I’ll see you’, but for some people it will be.”
Discussing the impact on their daughter, Mr Ratcliffe said: “She certainly has asked frequently ‘why did this happen to my mummy?’ And there’s not an easy way of explaining any of that.
“But at the same time, this is the reality she’s grown up with. If she’d been six when this started, in some ways, it would be more shocking for her.”
He added: “And now, what she’s had is a long series of her parents making promises that mum is coming home and not keeping them.”
During the call with Mr Rouhani, the PM also raised Iran’s breaches of its nuclear deal, the Joint Comprehensive Plan of Action (JCPOA).
The No 10 spokesman said Mr Johnson “stressed that while the UK remains committed to making the Iran nuclear deal a success, Iran must stop all its nuclear activity that breaches the terms of the JCPOA and come back into compliance”.
“He stressed the importance of Iran seizing the opportunity presented by the United States’ willingness to return to the deal if Iran comes back into compliance.
“The Prime Minister underlined the need for Iran to cease wider destabilising activity and be a positive force in the Gulf region.”
Amnesty International UK said the British ambassador in Tehran should visit Ms Zaghari-Ratcliffe ahead of another expected court hearing on Sunday to show “maximum solidarity” with her.
Kate Allen, director at Amnesty International UK, said: “Nazanin has already been convicted once after a deeply unfair trial before a Revolutionary Court, so of course we’re extremely concerned about this hearing.”
She said the Iranian authorities “systematically violate fair trial rights” and it was of the “utmost importance” that UK officials are granted permission to attend any court hearings.
Downing Street has previously said that officials have been denied access to legal proceedings because Iran does not recognise dual nationality.