French citizen Benjamin Briere is still in an Iranian jail despite his acquittal, his lawyer said on Thursday.
Mr Briere was detained in May 2020 and later sentenced to eight years in prison for espionage.
His lawyer Philippe Valent said the Iranian appeals court had cleared his client of all charges and ordered his release on February 15, AFP reported.
But Iran's Revolutionary Guards, the branch of the security forces entrusted with the preservation of the regime, “are keeping him imprisoned despite this declaration of innocence”, Mr Valent said.
“His release was prevented at the last moment,” he said.
Mr Briere has been on hunger strike for a month. He is being held in a prison of Vakilabad in the eastern city of Mashhad, where he is “exhausted physically and mentally”, Mr Valent said.
Mr Briere is one of several foreigners Iran has jailed in a strategy of hostage-taking to extract concessions from the West, campaigners say.
The French Foreign Ministry told AFP it was “concerned about the situation of Mr Briere”.
“France is calling for the immediate release of all French nationals,” it said.
Charges called 'fiction'
Mr Valent has previously described the espionage charges against his client as “fiction”.
He said on Thursday their “arbitrary nature” was “blatantly obvious”.
A copy of the appeals court judgement stated that Briere “was not present in the forbidden and sensitive places of the country and did not take any pictures or videos” of those places.
He was “only a foreign tourist” whose “personal items and equipment were not used for espionage”, the judgement stated.
“There is not sufficient evidence to establish a crime, and the verdict is issued for the acquittal of Mr Benjamin Briere,” it read.
“This situation is completely incomprehensible,” said Mr Briere's sister, Blandine.
She said the family had decided not to report the appeals court's verdict earlier in the hope that the situation might be resolved quietly.
Another detainee in Iran, 64-year-old Franco-Irish citizen Bernard Phelan held since October 1, in January suspended a hunger strike that included refusing water, at the request of his family, who feared for his life.
Mr Phelan, a Paris-based travel consultant, was arrested while travelling and is being held in Mashhad.
Iran accuses him of anti-government propaganda, a charge he has denied.
Six French nationals are currently held in Iran.
The ministry has repeatedly condemned Iran for what it calls “hostage diplomacy”.
Overall, Iran is holding at least 17 Western nationals, most of whom have dual nationality. They are described by their supporters as innocents used by Tehran as levers for negotiation.
Iran is one of the world's most sanctioned countries, with Western powers over the past four decades using a punishing array of measures in a bid to push change.
France on Thursday called developments in Iran's nuclear programme “very concerning” after the UN nuclear watchdog reported finding uranium particles enriched just under the 90 per cent needed for an atom bomb.