The lifetime of the nuclear reactors, originally intended to stop operating in 2015, was extended by the Belgian legislature that year to continue producing electricity through 2025.

Belgian environmental associations Inter-Environnement Wallonie and Bond Beter Leefmilieu brought a case before the Constitutional Court of Belgium asking that law be annulled as it was decided without an environmental assessment or public participation.

"The Belgian law extending the operating life of nuclear power stations Doel 1 and Doel 2 was adopted without the required environmental assessments being carried out first," the European Court of Justice ruled.

"It is not however excluded that the effects of the law on extension may provisionally be maintained where there is a genuine and serious threat of an interruption to electricity supply," the court said.

Doel 1 and 2, located near Antwerp and operated by Engie Electrabel, are Belgium's smallest of seven nuclear reactors, with capacities of 445 and 433 megawatts respectively.

(Reporting By Jan Strupczewski and Daphne Psaledakis)