The money and gifts totalled 320 million yen over seven years from 2011 and came from the late deputy mayor of Takahama town, where Kansai Electric (KEPCO) has a nuclear plant.

"I deeply apologise for causing great concern to stakeholders and the public," KEPCO president Shigeki Iwane told reporters.

The admission came after investigations by tax authorities into the deputy mayor revealed the payoffs.

"We were afraid our relationship with the local government would be damaged" if the gifts and money were rejected, Iwane said.

"We had in mind to return them at the right time. We knew they were expensive gifts and we had to return them one day."

But Iwane added he "would not reveal the details of who received what and when as the information is private," saying some of the gifts had already been returned.

According to local media, tax agency investigations found that former deputy mayor Eiji Moriyama received a 300-million-yen commission from a local construction company that was hired for projects at the Takahama nuclear power plant.

Moriyama reportedly told authorities he had decided to give KEPCO officials the money in the form of cash and gifts as a token of his appreciation.

It was not immediately clear if KEPCO, which runs the Takahama nuclear plant with four reactors in central Fukui prefecture, would face sanctions over the incident.

Before Iwane's press conference, Trade and Industry Minister Isshu Sugawara said a "thorough probe" was necessary.

"If (the news) is true, it's an extremely egregious and serious situation. It could shake the trust of communities where nuclear plants are located."