Incarcerated in May 2020, Briere was sentenced to eight years in prison for espionage.
He is one of several foreigners who campaigners say Iran has jailed in a strategy of hostage-taking to extract concessions from the West.
His France-based lawyer, Philippe Valent, said in a statement sent to AFP that an Iranian appeals court had cleared his client of all charges and ordered his release on February 15.
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,” he said.
“His release was prevented at the last moment,” he told AFP.
Held in the prison of Vakilabad in the eastern city of Mashhad, Briere is continuing a hunger strike which he started a month ago, and is “exhausted physically and mentally,” Valent said.
Valent, who had previously described the espionage charges against his client as “fiction,” said Thursday their “arbitrary nature” was “blatantly obvious”.
He said Briere’s family was demanding his immediate release.
“This situation is completely incomprehensible,” Blandine Briere, his sister, told AFP.
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.
Phelan, a Paris-based travel consultant, was arrested while travelling and is being held in Mashhad in northeastern Iran.
Iran accuses him of anti-government propaganda, a charge he has denied.
Six French nationals are currently held in Iran.
The French foreign ministry has repeatedly condemned Iran for what it calls “hostage diplomacy.”