Anoosheh Ashoori - a British national who has been held in Iran for more than four years - has started a hunger strike.
The retired civil engineer has been jailed in Evin prison since August 2017.
He starts his hunger strike as he approaches his 68th birthday after a lack of help from the Foreign Office his daughter Elika Ashoori says.
"My father who has been held hostage in Iran for the past four and a half years has decided to join a worldwide hunger strike," she said on Twitter.
"We are extremely concerned for his physical health as he approaches his 68th birthday."
"But having failed to see any progress in the FCDO's efforts to bring about his release and no signs the welfare of hostages currently held by Iran is the priority of their respective US, European and UK governments he will begin his hunger strike in bringing awareness of hostages plight in Iran."
An FCDO spokesperson has said: “Iran’s continued detention of Anoosheh Ashoori is wholly unjustified.
"We call on Iran to end his suffering and allow him to return home to be reunited with his wife and children.
“Iran should free all those British nationals unfairly detained in Iran.
"The Foreign Secretary, Minister Cleverly and senior officials consistently raise the cases of Nazanin Zaghari- Ratcliffe, Anoosheh Ashoori and Morad Tahbaz with the Iranian authorities and will continue to do so.”
Mr Ashoori was given a 10-year jail sentence for allegedly spying, and has had two brief furloughs before returning to jail.
It's not the first time he has been on a hunger strike, but he has since had Covid in prison,
He was told he lost his bid for release on the same day Nazanin Zaghari-Ratcliffe was told she had lost her appeal against a one-year further sentence for alleged propaganda activities against the regime.
Mr Ashoori flew out to Iran on a routine visit to see his mum. But he has since been held on spying charges.
Last winter Richard Ratcliffe went on a hunger strike for three weeks, while camping outside the Foreign Office.
Mr Ashoori’s son, Aryan, joined Mr Ratcliffe in a strike overnight urging the Government to do more.
