Belgium's Constitutional Court gave hope Friday to relatives of an aid worker jailed in Iran, rejecting a legal challenge to a law allowing prisoner swaps with Tehran.

The decision was a blow to the Iranian opposition group that brought the case, which had objected that the law would reward Iran for taking hostages by allowing it to swap them for jailed agents.

But the Belgian government and the family of Olivier Vandecasteele will welcome the decision, which could see the 42-year-old -- jailed in Iran for 40 years for spying -- come home.

It could also be good news for Iranian official Assadollah Assadi, jailed in Belgium after he was convicted for masterminding a plot to blow up a 2018 opposition event outside Paris.

Belgian Prime Minister Alexander De Croo's government believes cutting short Assadi's 20-year sentence and sending him home is the only way to secure Vandecasteele's release.

Critics of the policy, including the exiled Iranian group that brought the case, argue that the prisoner swap law will leave Belgian citizens prey to future Iranian hostage-taking.

The court, while rejecting moves to overturn the Belgian prisoner transfer law, did impose certain conditions that could complicate future detainee swaps.

"The government, when it makes a transfer decision, must inform the victims of the actions of the convict concerned so that they can effectively have its legality reviewed," it said.

This would mean, for example, that the relatives of those killed in a future attack could challenge a convict's release to Iran.

The National Council of Resistance in Iran (NCRI), an exiled Iranian political party, was the target of Assadi's 2018 bomb plot, for which he was convicted on terrorism charges.

It has challenged the Belgian law -- tailor-made to secure Vandecasteele's release -- in several courts, even as Vandecasteele's loved ones mobilised to support it.