The Government is "urgently" investigating reports that a British-Iranian woman detained in Iran for more than two months is to face charges of trying to cause a "soft toppling" of the country's government.
The accusations, reported by Iran's state-run news agency on Wednesday, mark the first official acknowledgement of the detention of Nazanin Zaghari-Ratcliffe.
Ms Zaghari-Ratcliffe, who works for the Thomson Reuters Foundation, the charitable arm of the news agency, was stopped on April 3 at Imam Khomeini airport in Tehran with her daughter Gabriella as she tried to return to the UK after a family holiday.
In a statement the UK Foreign Office said: "We are urgently seeking information from the Iranian authorities on the reported accusations being made against Ms Zaghari-Ratcliffe."
It added: "We have raised this case repeatedly and at the highest levels and will continue to do so at every available opportunity. We have also been supporting Mrs Zaghari-Ratcliffe's family since we were first made aware of her arrest."
Iran's Revolutionary Guard alleged Ms Zaghari-Ratcliffe, 37, had participated in the "design and implementation of cyber and media projects to cause the soft toppling of the Islamic Republic."
Gabriella, who turned two last week, had her British passport confiscated by the Iranian authorities and is staying with her grandparents while her mother is held 621 miles (1,000km) away in Kerman Prison.
Earlier this month Ms Zaghari-Ratcliffe's husband, Richard Ratcliffe, from West Hampstead, left a birthday card on the doorstep of the Iranian embassy in London and sang Happy Birthday over Skype to mark Gabriella's birthday.
Under Iranian law only her father or mother can bring her home.
In a bid to reunite the family, thousands of people have sent cards to Iranian embassies across the world.
A Change.org petition has topped 764,000 signatures, and Mr Ratcliffe hopes to have sent to Iran's supreme leader Ali Khamenei once it passes one million names.
Iranian authorities have previously said Mrs Zaghari-Ratcliffe's detention was over an issue of "national security".