The Nuclear Regulation Authority (NRA) introduced tougher new rules in 2013 requiring utilities to be able to maintain nuclear reaction cooling facilities by remote control if necessary.

The regulations aim to prevent radioactive leaks if nuclear plants came under attack -- for example from terrorists piloting planes into them.

Companies given permission to restart operations after the disaster -- the worst nuclear accident since Chernobyl in 1986 -- had five years to conform to the stricter regulations.

But several firms have warned they will not meet these criteria and the NRA said after a meeting earlier Wednesday it would no longer push back the deadline as it has done in the past.

"There is no need to extend the deadline and nuclear facilities have to stop operations if the operators fail to meet it," an NRA official told AFP.

A reactor at the Sendai power plant in western Japan could be the first to be suspended if the operator -- Kyushu Electric Power -- fails to finish work by the deadline next March, said the official.

He added that several other nuclear reactors were also at risk of being shut down.

The news hammered the share prices of major utilities, with Kansai Electric plunging nearly eight percent and Kyushu Electric diving more than five percent.

Before the Fukushima disaster, Japan relied on nuclear for roughly 30 percent of its electricity.

But this declined to less than two percent after the crisis, as reactors were suspended for emergency safety checks, with many of them unable to resume operations under the stricter rules.