Belgium's Constitutional Court has rejected a request to annul a prisoner exchange treaty with the Islamic Republic that can lead to the release of an Iranian diplomat. 

The constitutional court said in a press release Friday that "The Court rejects the action for annulment," a decree that could lead to Iranian diplomat Assadollah Assadi, convicted of terrorism, being swapped for Belgian aid worker Olivier Vandecasteele, held hostage in Iran. 

“The Court rejects the appeal against the law assenting to the Belgian-Iranian treaty, but the victims of a convicted person must be informed of his transfer so as to be able to submit this for the review of the legality by a judge,” read the statement. 

The court added that “This balancing act must be made in a concrete case and on a case-by-case basis, and must be open for judicial review. It is up to the competent judge (in principle the Court of first instance), with respect for the separation of powers, to review the legality of the decision of the Government authorizing the transfer of a person detained in Belgium towards a foreign State of which it is a national.”

The temporary ban on the extradition of the convicted diplomat was announced by the Brussels Court of Appeal late in July following numerous complaints after the Belgian Parliament ratified a controversial prisoner swap treaty with Iran on July 20. The Iranian exiled opposition group Mujahedin-e Khalq Organization (MEK) mounted a fierce campaign against the deal, challenging the possible extradition. "The court has now declared the plaintiffs' question unfounded," said lawyer Khloë Georgiev, who represents Vandecasteele. In June, Belgian MPs ratified a deal to allow the exchange. At the time, Prime Minister Alexander De Croo argued that it was the only way to get Vandeesteele back home safely.

The Islamic Republic’s judiciary sentenced Vandecasteele, who was detained in 2022, to 40 years in prison and 74 lashes for alleged “spying and cooperation with the United States, money laundering and smuggling $500,000 out of Iran.” Vandecasteele, 41, has served in various international humanitarian organizations since at least 2006, including, Médecins du Monde, Norwegian Refugee Council (NRC), and Relief International. He worked in Iran for humanitarian organizations for more than six years and left the country. Later, he was lured back by “a girlfriend” and was detained in February 2022. A spokesman for Vandecasteele’s family said in December that he should serve up to 28 years in prison. 

In a statement released late in February, Amnesty International said Iranian authorities are subjecting Vandecasteele to enforced disappearance, torture and other ill-treatment, urging all states whose nationals are or have been detained at any point in Iran to promptly examine whether the deprivation of liberty amounts to an act of hostage-taking, and if so, take all appropriate measures to ensure accountability. Currently, at least 16 foreign citizens, most of whom dual nationals, are detained in the prisons of the Islamic Republic.

Assadi, 50, a former attaché at the Iranian embassy in Austria, was convicted of plotting to bomb a gathering of the MEK near Paris on June 30, 2018. The only Iranian diplomat ever brought to trial in Europe for direct involvement in terrorism was arrested in Germany, where he did not enjoy diplomatic immunity, while he was on holiday. German authorities later extradited Assadi to Belgium.

Some Belgian lawmakers voiced concern that the prisoner exchange treaty might lead to "hostage diplomacy" and put other Belgians at risk of detention.

On Monday, February 27, Belgian Foreign Minister Hadja Lahbib held a meeting with the Islamic Republic’s Foreign Minister Hossein Amir-Abdollahian hours before tens of participants at the UN Human Rights Council walked out when Akir-Abdollahian spoke at the 52nd session of the body at UHRC’s headquarters in Geneva.

The meeting between the Belgian and Iranian ministers drew heavy criticism by many activists and officials, including Belgian lawmaker Darya Safai, who pointed out that “The Belgian government goes along with hostage diplomacy.” Lahbib herself tweeted about the release of Vandecasteele, who was probably the main reason for the meeting.