A Belgian court convicted Assadi in 2021 of providing explosives to a Belgian-Iranian couple and plotting a foiled terrorist attack on a political event hosted by the National Council of Resistance of Iran in Paris in 2018. He is serving a 20-year prison sentence.

The letter was signed by 68 legal experts and former officials, including current and former EU judges and UN officials, and members of international human rights organizations.

It said that “releasing Assadi back to Iran would only fuel the culture of impunity that exists for Iran’s officials."

Belgium’s Parliament in July ratified a treaty with Iran that would permit prisoner exchanges between the two countries. In February, Iranian security forces detained a Belgian aid worker, Olivier Vandecasteele, 41, on charges of espionage.

In accordance with the treaty, Belgium could release Assadi into Iranian custody, in theory to serve the remainder of his sentence in his home country, in exchange for the release of Vandecasteele. However, the treaty also permits both sides to grant amnesty to citizens who are exchanged.

Among those who signed the letter are European and North American political and legal luminaries, including three former judges of the General Court of the European Union, a former president of the Organization for Security and Co-operation in Europe’s Court of Conciliation and Arbitration, a former president of the Constitutional Tribunal of the Republic of Poland, a former foreign minister of Canada, a former attorney general of Portugal, France’s former human rights ambassador, and a former US ambassador to the UN Commission on Human Rights.

“Allowing Assadi to serve the remainder of his 20-year sentence in Iran, the state which was responsible for the attempted terrorist bombing, would make a mockery of the rule of law and foster further impunity for the Iranian government and its officials involved in terrorism and crimes against humanity,” the letter said.

It warned that a “dangerous precedent” would be set if Belgium sends Assadi back to Iran.

“Transferring Assadi to Iran would effectively free him from serving his sentence and would set a dangerous precedent and seriously weaken the rule of law in Europe,” it said.

“It would encourage more Iranian terrorism on EU soil and reassure Iranian officials that they could evade responsibility for major international crimes. Belgium would bear heavy responsibility in this regard.”

The London-based Iranian dissident group Justice for the Victims of the 1988 Massacre in Iran said it strongly opposes the return of Assadi to Iran and urged the Belgian government to resist such a move.

Tahar Boumedra, one the signatories to the letter, is a former head of the UN Assistance Mission in Iraq’s Human Rights Office and former representative of the UN High Commissioner for Human Rights in Iraq. He told Arab News that Belgium must adhere to the principles of international law and refuse to return Assadi to Iran. Doing so, he added, would be a big mistake that the Iranian regime would see as an incentive to use the taking of hostages as a political tool.

“Our open letter is meant to remind the Belgian government to abide by its obligations under international law, particularly the provisions of the International Convention for the Suppression of Terrorist Bombings, signed and ratified by Belgium on May 20, 2005,” Boumedra said.

The National Council of Resistance of Iran, which is active in exile across Europe and the US, is the political arm of Iranian opposition group Mujahedin-e Khalq, which seeks to topple the regime in Tehran and establish a democratic republic.