Belgium's Supreme Court has prohibited the implementation of a prisoner exchange treaty with Iran.
Opponents of the exchange were concerned that the deal would facilitate a bomb-plot mastermind's return to Iran.
Assadollah Assadi, who was an Iranian diplomat, was sentenced to 20 years in prison, convicted of supplying explosives to bomb a meeting in Paris of Iranian exiled opposition.
In exchange for his release, Iran was set to release aid worker Olivier Vandecasteele, detained in Iran since February.
The Belgian court has suspended the exchange treaty for at least three months until a final ruling.
The court had ruled that Assadi had masterminded the plot to attack the exiled Iranian opposition under the cover of a diplomat to Austria and therefore had no immunity in Belgium.
Iran wants Belgium to recognize Assadi's diplomatic immunity.
Meanwhile, aid worker Vandecasteele says he is being treated inhumanely in his Iranian prison and has launched a hunger strike and his family members are concerned about his failing health.