Among the three, Chugoku Electric Power Co was hit with the biggest fine, of more than 70 billion yen ($514 million), the Nikkei business daily said.

Chubu Electric Power Co was fined 27.5 billion yen, the Nikkei said, while Kyushu Electric Power Co said it had received notice for 2.7 billion yen.

In separate statements, all three confirmed they could be fined by the Fair Trade Commission. Chugoku did not disclose the amount, while Chubu Electric said it would incur a special loss of 27.5 billion yen ($201.52 million) in the third quarter, citing the penalty.

Regulators had investigated the companies for anti-competitive behaviour and concluded that they had formed a cartel and obstructed the liberalisation of the nation's electricity market, the Nikkei said.

Shares of Chugoku Electric sank 3.8%, Chubu Electric by 1.9% and Kyushu Electric by 2.2%, underperforming a near 1% rise in the benchmark Nikkei average.

Japanese utilities are suffering from rising input costs amid high global energy prices exacerbated by the yen's weakness and are seeking price hikes of as much as 30% to 50% in the coming months.

The Nikkei previously reported that Kansai Electric Power Co had also been part of the scheme but that it had probably avoided the penalty by reporting the matter to the antitrust regulator and taking advantage of a leniency policy. A Kansai Electric spokesperson had said it was fully cooperating with the regulator.

($1 = 136.1900 yen)

($1 = 136.3000 yen)

(Reporting by Mariko Katsumura; Editing by Shri Navaratnam & Simon Cameron-Moore)