The sister of an Iranian-American environmentalist made a plea for his release after two dual nationals including British-Iranian aid worker Nazanin Zaghari-Ratcliffe were allowed to leave the country this week.
Taraneh Tahbaz told Reuters that her brother, Morad Tahbaz, who also holds British citizenship, had been taken back to prison on Friday after being released on furlough on Wednesday.
Shortly afterwards, a spokesman for Britain's foreign ministry said it had been told by Iran that Tahbaz, 66, had been taken back to Evin prison in order to fit an ankle bracelet and that he would be allowed out in coming hours.
Tahbaz was arrested in 2018 and sentenced to 10 years in prison for "assembly and collusion against Iran's national security" and working for the United States as a spy.
On Thursday, Zaghari-Ratcliffe - who is employed by the Thomson Reuters Foundation - and Anoosheh Ashoori arrived in Britain from Iran. Zaghari-Ratcliffe was arrested in 2016 and spent most of the time after that in prison. Ashoori received a 10-year sentence in 2019.
Iran's semi-official Fars news agency said they were freed after Britain repaid a historic debt relating to the purchase of military tanks.
Tahbaz's sister, speaking before the comments from Britain's foreign ministry, said the way that her brother had been left in Iran was hugely worrying.
"There are so many political things going on in the world that his case might just be swept under the rug," she said in an interview in Madrid where she lives.
"We are hoping that the American authorities will react. We are hoping that British authorities will also come forward and help out because he is a British subject too, or are they only listening to the Iranians who say that he is an American?"
Taraneh Tahbaz said her brother was suffering from cancer and had contracted COVID-19 twice.
"My brother's condition is very precarious right now and I urge and plea that the American authorities put all their efforts behind getting him out," she said.
"If the UK authorities can bring themselves to help, it would do a great deal."