European Union officials have accused Mastercard of overcharging consumers with the fees the company charges.
“We currently suspect Mastercard is artificially raising the costs of card payments,” said the EU statement.
Those fees harm consumers and retailers in the EU, competition commissioner Margrethe Vestager said.
The credit card company said it was “working with the European Commission on the issue” and was preparing a formal response.
“Throughout this procedure we have kept the needs of both consumers and merchants in mind,” Mastercard said.
The EU is focusing on what is known as ‘interchange fees’ – the fees retailers pay banks to process card payments.
Officials say they are concerned that Mastercard violates competition rules by requiring retailers to pay artificially inflated fees to process credit card transactions.
They say this practice leads to higher prices for retailers in the EU, which they then pass on to consumers.
The charges from the EU are part of a long-running investigation.
The maximum fine that the EU can impose for antitrust infringements is 10% of the company’s annual sales, according to the EU.
A separate investigation into Visa’s interchange fees is also being carried out by the EU.