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President on Sunday ratified a prisoner exchange treaty with , official media said, after criticism in the European country that it could free a bomb-plot mastermind.

Belgium's opposition has alleged the treaty was "tailor made" to permit the release of Assadollah Assadi, an Iranian diplomat sentenced last year to 20 years in prison.

An Antwerp court convicted him of supplying explosives for a foiled 2018 bomb plot, targeting a meeting outside Paris of Iran's exiled opposition.

The Belgian government said the deal was the only way to release aid worker Olivier Vandecasteele, who was detained in Iran in February.

He is one of several Western nationals – most of whom also hold Iranian passports – detained in Iran, in what foreign based activists have said is a bid to extract concessions.

Iran's parliament approved the treaty in August, and late last month another body, which determines whether laws are constitutional, gave its assent before the final step of presidential endorsement.

Raisi "ratified the law on the transfer of prisoners between the Islamic Republic of Iran and the Kingdom of Belgium, permitting it to take effect", state news agency IRNA reported.

Foreign critics of the treaty, who also include Iran's vocal exiled opposition, argued that Belgium backed down in the face of what is in effect hostage-taking by Tehran.

The Assadi case sparked a furious reaction in .

The Belgian court ruled that he had masterminded the bomb plot, under diplomatic cover as an envoy to Austria – and thus had no immunity in Belgium.

Tehran has demanded that Belgium recognise Assadi's diplomatic status and release him.

Belgian judges in July granted a request from opponents of the prisoner swap treaty to block any attempt to send Assadi home, until the decision can be challenged in court.