The European Commission has fined Mastercard 571 million euros, or around $650 million, for breaching antitrust rules by raising payment-processing fees artificially, leading to higher prices for retailers and consumers.

The penalty, announced on Tuesday after a six-year investigation by European antitrust regulators, involves the fees banks charge merchants when purchases are made with credit cards.

Mastercard, regulators said, forced merchants to use only banks in their home countries when processing payments, a requirement that prevented them from shopping for lower fees at banks in other European countries.

The retailers passed the cost of the higher fees on to consumers, leaving them to pay more for their purchases, the commission said .