The Iranian authorities sentenced Belgian aid worker Olivier Vandecasteele to 28 years in prison, according to a spokesman for his family.
Vandecasteele, 41, was arrested in Iran at the end of February and was once held in Evin prison over "fabricated series of crimes" on suspicion of espionage.
Belgium and Vandecasteele's family stresses that he is innocent and is the "victim of the shady international game," insisting that he is being held only as a hostage in Tehran's efforts to pressure Belgium to release an Iranian agent convicted of terrorism.
"The family is devastated," family spokesman Olivier Van Steirtegem told AFP after the government informed them of the news.
"Can you imagine? If there's no solution, he could stay in prison until 2050. He'll be almost 70," said the spokesman, urging Belgium to find a way to revive a prisoner swap treaty.
Van Steirtegem said that Vandecasteele's family had been invited to meet Belgium's Prime Minister Alexandre de Croo and several ministers.
Justice Minister Vincent Van Quickenborne received a call from his Iranian counterpart communicating the court's verdict, but that they had no details on the charges.
Last week, Belgium's Constitutional Court suspended the controversial treaty, pending a final ruling on its legality within three months.
Opponents of the Iranian government rejected the deal, which they say is "tailor-made" to allow the release of Assadollah Assadi, an Iranian diplomat sentenced to 20 years in prison after an Antwerp court found him guilty of delivering explosives to a couple from Belgium who intended to travel to Paris to target a meeting of the Iranian exiled opposition.