(Reuters) -Payments giant Visa said on Wednesday the European Commission (EC) had opened a preliminary investigation into fees charged to retailers.

Reuters had exclusively reported earlier this month that EU antitrust regulators were investigating whether fees charged by card giants Visa and Mastercard had a negative impact on retailers.

The EC informed Visa about the investigation on Aug. 30, it said, adding that the company was cooperating with the regulator in connection with the investigation.

Last month, Mastercard also disclosed that the EC had sought documents tied to a broader investigation into alleged anti-competitive behavior in the European Union.

Visa and Mastercard have long dominated the market for payment cards and often been subject to scrutiny and allegations of a duopoly.

Additionally, Visa said that the EC informed it on Oct. 1 that a separate investigation into the company's incentive agreements with clients had been closed.