A price-fixing scandal at Japan’s biggest casualty insurers is escalating following a media report that the suspected collusion is likely to be wider than initially believed.

Shares of Tokio Marine Holdings Inc., MS&AD Insurance Group Holdings Inc. and Sompo Holdings Inc. dropped more than 4% on Wednesday after Nikkei reported that more corporate clients may have had premiums fixed.

The allegations first came to light in June, when the insurers were said to have tried to set premiums for railway operator Tokyu Corp. The Financial Services Agency ordered them to submit a report on the matter, amid questions over whether the suspected collusion was an isolated incident or systematic.

Nikkei, citing people familiar with the matter, reported on Tuesday that dozens of suspicious cases have emerged during the insurers’ investigations. They are alleged to have colluded in determining premiums for companies including refiner Eneos Corp. and East Japan Railway Co., the newspaper said.

A Tokio Marine spokesman said the company can’t comment on specific clients or contracts, while adding that it continues to look into the matter. Representatives for Sompo and MS&AD declined to comment, citing the ongoing investigation.

The possibility of regulatory action is weighing on the shares of the property-casualty insurers, said Masayuki Otani, chief market analyst at Securities Japan Inc. “Foreign investors are especially averse to this kind of scandal and their wait-and-see stance is likely pushing down stocks,” Otani said.

Japanese insurers also face regulatory scrutiny on another matter. Finance Minister Shunichi Suzuki said this week that the FSA has issued a report order on a unit of Sompo and six other insurers in connection with alleged insurance fraud by used-car dealership Bigmotor Co. The minister said the FSA will check the facts around Sompo seconding its employees to Bigmotor and their business relationship.

Sompo said last week that it’s preparing to end its contract with Bigmotor, which has admitted to deliberately damaging vehicles and padding insurance claims.