BRUSSELS, March 3 (Reuters) - Belgium's Constitutional Court on Friday rejected a request to annul a prisoner exchange treaty with Iran that could lead to a convicted Iranian diplomat being swapped for a jailed Belgian aid worker.

Belgian lawmakers cleared the treaty in July but it has been held up due to legal challenges from an exiled Iranian opposition group.

"The Court rejects the action for annulment," the constitutional court said in a press release.

However, the judges also specified that the victims of any detainee being proposed for transfer must have the right to contest the specific case in court.

"Thus, when the Government takes a decision to transfer, it must inform the victims of the relevant convicted person in such a way that they can effectively seek a review of the legality (of the transfer)," it said.

Aid worker Olivier Vandecasteele, arrested while on a visit to Iran in February 2022, was sentenced in January to 40 years in prison and 74 lashes on charges including spying that Brussels has denounced as fake.

His distraught family has appealed to the government to do its utmost to get him freed.

Iran has called for the release of Assadollah Assadi, sentenced to 20 years in prison in Belgium in 2021 over a foiled 2018 bomb plot. His was the first trial of an Iranian official for suspected terrorism in Europe since Iran's 1979 revolution.

The exiled National Council of Resistance of Iran (NCRI), whose rally near Paris had been the bomb plot's target, has insisted that Assadi remain in jail.

The Islamic Republic has called the Paris attack allegations a "false flag" stunt by the NCRI, which it in turns considers a terrorist group.

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

It is not clear when a prisoner exchange might happen.

Reporting by Philip Blenkinsop, Andrew Gray and Geert De Clercq; Editing by Sharon Singleton