Belgium’s government has made a formal request to Iran for the return of a jailed aid worker after the signing of a controversial prisoner exchange treaty.

Olivier Vandecasteele has been held in the Islamic Republic since last year on spurious charges of “spying and cooperation with the United States, money laundering and smuggling $500,000 out of Iran.”

An Iranian court sentenced the humanitarian worker to 40 years in prison and 74 lashes, it was announced in January.

Vandecasteele, 41, has been employed by Médecins du Monde, Norwegian Refugee Council (NRC), and Relief International. He worked for humanitarian organizations in Iran for more than six years before leaving the country, but was lured back by “a girlfriend” and was detained in February 2022.

He has been subject to torture, according to Amnesty International, and is being held in solitary confinement in a windowless basement cell without access to adequate healthcare and fresh air.

Belgian Foreign Minister Hadja Lahbib confirmed on Tuesday that she had notified her Iranian counterpart Hossein Amir-Abdollahian of the transfer request.

In a social media post, she said: "I condemned his detention conditions and requested a visit by our ambassador to Iran."

A swap is being discussed between Brussels and Iran for Iranian official Assadollah Assadi, jailed in Belgium for masterminding a plot to bomb an Iranian opposition event near Paris in 2018.

In response to a legal challenge by an Iranian opposition group, the potential prisoner exchange was held up for months.

Belgium's Constitutional Court rejected a request to annul the prisoner exchange treaty with the Islamic Republic in March.