A Japanese court issued an injunction to halt operations at Kansai Electric Power's Takahama No.3 and No.4 nuclear reactors, the operator said, siding with local residents worried about the safety of the plant.
The order, by the Otsu District Court, will take immediate effect and lead to the shutdown of Takahama No.3 reactor, which restarted in late January, Jiji news agency said. This is the first injunction issued in Japan to halt a nuclear plant that is under operations, said Jiji and national broadcaster NHK.
Kansai Electric had also been working to restart Takahama No.4 reactor this month after an unplanned shutdown due to a technical problem.
The move could potentially throw government energy policy into disarray, with the nuclear industry only recently starting to get reactors back online amid widespread public scepticism after the meltdowns at Fukushima in 2011. It also comes as Japan's energy market embarks on the biggest reforms in its history.
However, Japanese lower courts sometimes hand down contentious verdicts that are then overturned by higher courts, where judges tend to be more attuned to political implications, judicial experts say.
Kansai Electric issued a statement saying it would not accept the verdict and would quickly appeal the injunction.
"This is a wake up call for nuclear industry and the government. They can no longer take for granted that the judiciary will follow the old ways," Mutsuyoshi Nishimura, a former Japanese government official and chief climate change negotiator said after the decision.
Japan's chief government spokesman Yoshihide Suga said after the verdict that there was no change in Tokyo's stance on safety at the Takahama reactors or in its policy of promoting the restart of reactors that meet new safety standards imposed after Fukushima.
Kansai Electric planned late last month to lower power fees it charges to customers from May 1 to pass on fossil fuel cost savings from the restart of Takahama No.3 and No.4 reactors, but the verdict could force Japan's No.2 power utility in revenues to scrap that plan.
The Takahama reactors, on the coast of Fukui prefecture in western Japan, had met beefed-up safety standards set by Japan's nuclear regulator last year.
