Anoosheh Ashoori was arrested in Iran in 2017 and sentenced to 10 years in prison after being accused by the Iranian government of spying, charges he denies.
He is one of eight known dual-nationality British citizens to have been arrested and detained in Iran in recent years. That number includes Nazanin Zaghari-Ratcliffe, convicted under spying charges that have been widely denounced as baseless.
Also detained in Iran is Hammersmith-born Morad Tahbaz. The conservationist was arrested in 2018 and had been diagnosed with cancer.
Nizar Zakkar is a Lebanese businessman and US resident who spent four years in the same prison under similar charges.
The US government has claimed such detentions amount to hostage taking for ransom, an allegation denied by Iran.
The BBC’s Darragh MacIntyre investigates these arrests and suggestions there may be more British citizens in similar circumstances.
UK viewers can watch
Hostage in Iran on Monday 24 August on BBC One at 19:30 BST.