The industry ministry said Friday it ordered four major Japanese utility groups to improve their operations after they were found to have formed electricity sales cartels.

Kansai Electric Power Co., Chugoku Electric Power Co., a sales unit of Chubu Electric Power Co., Kyushu Electric Power Co. and its sales unit are all required to submit their improvement plans by Aug. 10 to prevent a recurrence of a similar incident.

The Ministry of Economy, Trade and Industry's monitoring committee on gas and electricity businesses concluded that their actions hinder liberalization of the electricity retail market and the development of the electricity sector.

Multiple instances of misconduct by major power companies have put them under the spotlight lately. Kansai Electric and Kyushu Electric were also slapped with separate business improvement orders in April, along with three other utilities, over unauthorized access to customer information of rival electricity retailers.

In March, Chubu Electric, Chugoku Electric and Kyushu Electric were ordered to pay a total of ¥101 billion ($731 million) in fines by the Japan Fair Trade Commission for violating the antimonopoly law by forming the cartels.

The amount was the highest ever imposed by the antitrust watchdog in the country.

Kansai Electric, which had formed a cartel with each of the other three utilities, was exempted from paying the fine after voluntarily reporting the violation to the watchdog before the probe began.

Japan has opened up the electricity market in stages since 2000 to end the decadeslong monopoly of regional power companies and help lower electricity bills by promoting competition.

The ministry also imposed a business improvement order on Kansai Electric in 2020 over a bribery scandal involving its executives.