The sister of an Iranian-American environmentalist made a plea for his release after two dual citizens including British-Iranian aid worker Nazanin Zaghari-Ratcliffe were allowed to leave the country this week.
Morad Tahbaz, who also holds British citizenship, was taken back to prison on Friday after being released on furlough on Wednesday.
He had been allowed to leave prison four years into a 10-year sentence under the agreement that led to Britain settling a £400 million ($527.2m) arms debt from the 1970s. But he was told he must remain in Iran as part of the deal.
On Saturday, his sister Taraneh Tahbaz said he had been returned to prison.
His family was said to be “devastated” that he had not been allowed to leave.
A spokesman for British Foreign & Commonwealth Office said it had been told by Iran that Mr Tahbaz, 66, had been taken back to Evin prison, where an ankle bracelet would be fitted, and that he would be allowed out in the coming hours.
“There are so many political things going on in the world that his case might just be swept under the rug,” Ms Tahbaz said.
“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?”
Mr 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 US as a spy.
On Thursday, prisoners Ms Zaghari-Ratcliffe, 43, and Anoosheh Ashoori, 67, were returned to the UK to be reunited with their families.
Ms Zaghari-Ratcliffe was arrested in 2016 and spent most of the time since then in prison.
Mr Ashoori received a 10-year sentence in 2019.
Ms Tahbaz said her brother had cancer and had contracted the coronavirus 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.”
The Iranians claimed Mr Tahbaz’s US citizenship made his case more complicated, UK Minister James Cleverly said.
Mr Tahbaz was part of a group of conservationists from the Tehran-based Persian Wildlife Heritage Foundation, which had been licensed to operate in Iran by the government in 2018.
A senior figure from the group died while interrogations were held at Evin prison in Tehran, said the US-based Centre for Human Rights in Iran.
They were convicted after a case based partly on a retracted forced confession.