The EU said Thursday that US credit-card giant MasterCard overcharged customers and retailers, having already found rival Visa at fault over fees levied on card payments.
The European Commission said in a preliminary statement of objections that it believed MasterCard was in breach of EU single market competition rule.
"Many consumers use payment cards every day, when they shop for food, clothes or purchase anything online. We currently suspect MasterCard is artificially raising the costs of card payments, which would harm consumers and retailers in the EU," EU Competition Commissioner Margrethe Vestager said.
"We have concerns both in relation to the rules MasterCard applies to cross-border transactions within the EU, as well as the fees charged to retailers for receiving payments made with cards issued outside Europe," Vestager said.
"MasterCard now has an opportunity to respond to our charges."
The Commission, which polices competition policy, has been investigating MasterCard and Visa for years over the fees they charge following complaints by customers and retailers alike that they were getting a raw deal.
It said Thursday that its main concern was over what are known as interchange fees, the charge for using a card and which varies from country to country.
MasterCard's rules meant banks could not offer lower interchange fees to retailers based in another country where the charge might be higher.
"As a result, retailers cannot benefit from lower fees elsewhere and competition between banks cross-border may be restricted, in breach of European antitrust rules."
In 2013, Visa cut its fees by up to 60 percent and agreed to "reform its rules to facilitate cross-border competition" following a similar Commission probe.
The sending of a formal statement of objections does not prejudge the outcome of the investigation but allows MasterCard to reply formally to the Commission charges.
The Commission can fine a company up to 10 percent of annual sales if it is found in breach of EU competition rules.