Japan's nuclear regulatory body decided Wednesday to effectively ban Tokyo Electric Power Company Holdings Inc. from restarting a nuclear plant on the Sea of Japan coast for around a year and a half after the complex was found to have serious safety flaws.

The Nuclear Regulation Authority agreed at its meeting to bar the company from transporting nuclear fuel stored at the Kashiwazaki-Kariwa plant in Niigata Prefecture or loading it into the reactors. The decision is a heavy blow to TEPCO, which saw the restart of the plant as a main pillar of its reconstruction plan following the nuclear crisis at its Fukushima plant in 2011.

File photo taken in September 2017 shows the Kashiwazaki-Kariwa nuclear power plant in Niigata Prefecture, operated by Tokyo Electric Power Company Holdings Inc. (Kyodo) ==Kyodo

The punitive measure will remain in effect until TEPCO's response to the incident is "in a situation where self-sustained improvement is expected," according to the regulator.

Facing huge compensation payments and other costs stemming from the nuclear disaster at the Fukushima Daiichi plant, TEPCO had been keen to resume operation of the Kashiwazaki-Kariwa plant to reduce its dependence on costly fossil fuel imports for non-nuclear thermal power generation.

The seven-reactor Kashiwazaki-Kariwa complex was known as one of the world's largest nuclear power plants with a combined output capacity of 8.21 million kilowatts when it was fully operational. Although the Nos. 6-7 reactors cleared safety screenings by the regulator in 2017, all seven reactors remain offline.

The nuclear plant was found to have been vulnerable to unauthorized entry at 15 locations since March last year because of defective intruder detection systems and backups, TEPCO and the regulator said earlier this month.

The regulator provisionally rated the breach at the plant to be at the worst level in terms of safety and severity, marking the first time it has given such an assessment.

NRA Chairman Toyoshi Fuketa said the regulator plans to receive TEPCO's analysis on the causes of the security breach incident and an improvement plan within the next six months and then conduct an investigation on the operator that is expected to last for at least a year.

"What is being questioned now is TEPCO's attitude regarding the protection of nuclear materials. TEPCO is not fit to transport fuel at Kashiwazaki-Kariwa," Fuketa said, adding the latest disciplinary order will immediately be effective in enhancing nuclear security.

A final decision of the measure will be made after TEPCO is given an opportunity to provide an explanation.

An executive of TEPCO said at a press conference in Fukushima Prefecture on Wednesday there is no room to raise an objection to the regulator's decision.

"TEPCO could not put into use the lessons and regrets from the Fukushima Daiichi accident," Makoto Okura, representative of the company's Fukushima Revitalization Headquarters, said.

Okura added TEPCO needs to "rebuild itself from its roots."

The regulator's decision comes as the Japanese government pushes to restart nuclear power plants deemed safe. The government sees that atomic power will remain one of the country's major power sources due to the instability of renewable energy.

Chief Cabinet Secretary Katsunobu Kato called it "extremely regrettable" that TEPCO will be slapped with the order. "It is important that TEPCO take drastic measures on management," the top government spokesman said at a press conference.

The NRA commissioners that took part in the meeting had also considered other disciplinary actions, including revoking TEPCO's license to operate the Kashiwazaki-Kariwa complex or ordering the halt of plant operations for up to a year.

It will be the first time the regulator has issued a corrective order for a commercial nuclear reactor, based on violation of the law regulating nuclear source material, nuclear fuel material and reactors.

In 2013, the regulator also effectively banned the restart of the Monju prototype fast-breeder reactor after its operator was found to have failed to inspect nearly 10,000 devices at the reactor in central Japan's Fukui Prefecture.

The Kashiwazaki-Kariwa plant has been plagued by security issues, including an incident in which an employee used a colleague's ID pass last September to enter the central control room without authorization.

Safety installations have also not been completed for the No. 7 reactor at the plant, and fuel loading that was originally scheduled to be conducted in March and April has been revised to "undecided."