A British-US national detained by Iran has gone on hunger strike, his sister has said.

Wildlife conservationist Morad Tahbaz, 66, who also holds Iranian citizenship, was taken back into custody after originally being allowed out on furlough on the same day last week that charity worker Nazanin Zaghari-Ratcliffe and retired civil engineer Anoosheh Ashoori were released and then allowed to return to Britain.

His sister, Tahrane Tahbaz, told Today on BBC Radio 4 that he was taken back into prison after just 48 hours.

“He was taken back to prison after 48 hours. He wasn’t really on furlough. It seemed more like a visit than a furlough.

“He didn’t have an ankle bracelet put on him. He was with security and, after 48 hours, he was taken back under security – back to the prison.

“We haven’t heard from him since and we have heard through a relative just a few hours ago that he’s been taken from the prison and he’s been taken to an undisclosed location and that he’s gone on hunger strike.”

Labour leader Sir Keir Starmer has called on the Government to “double down” on the case.

On a visit to Stevenage on Monday, he said: “It’s tragic that he is still there.

“We want to see everybody who’s in captivity in Iran back, you know, where they want to be with their families.

“I do say to the Government: double down on this. Of course it’s sensitive, of course it’s difficult, we understand that. But we all want to see this result just as quickly as possible.”

Mr Tahbaz’s sister said that their family were led to believe that he would be part of the same release deal as Mrs Zaghari-Ratcliffe and Mr Ashoori.

“Frankly, for four years we were led to believe that he would be part of the deal when it was made,” she said.

“And that’s what we were told. And the deal was made, the money was paid and he wasn’t part of the deal – and he’s still there – and we’re very worried.”

She said that the family feels “abandoned”.

“We feel very abandoned. His condition remains dire, and we just don’t know how long this is going to take.”

Last week, the UK said it secured Mr Tahbaz’s furlough, along with the release and return of the two other dual nationals.

This came after the Government finally agreed to settle a £400 million debt to Iran dating back to the rule of the Shah in the 1970s.

However, just two days later Mr Tahbaz was forced to return to Evin Prison.

The Foreign, Commonwealth and Development Office told the PA news agency on Monday that Morad has now been moved from the prison to a residential location in Tehran.

A FCDO spokesperson said: “Morad has now been moved from Evin prison to a residential location in Tehran.

“We have been in touch with Morad’s family throughout the day and continue to lobby the Iranian authorities at the highest levels to allow him to return home immediately, as the Iranian government committed to doing.

Mr Tahbaz, who has cancer, was arrested during a crackdown on environmental activists in January 2018.

He is a prominent conservationist and board member of the Persian Wildlife Heritage Foundation, which seeks to protect endangered species.

Mr Tahbaz was sentenced to 10 years in prison with his colleagues on vague charges of spying for the US and undermining Iran’s security.