The European Union has launched a competition investigation into some of MasterCard's payment fees.
The European Commission, the EU's executive arm that acts as the competition authority, said it has concerns that some of the US company's interbank fees and related practices may violate competition rules.
EU spokesman Antoine Colombani said that interbank fees vary greatly across the 27-nation EU, with a card payment of €50 in the Netherlands costing 10 cent, but up to eight times that in Poland.
The Commission said rules hindering merchants from enjoying better terms offered by banks elsewhere may violate competition standards.
It is also investigating fees charged to merchants when non-European card holders shop in Europe.
In 2007, the EU prohibited some of MasterCard's interbank fees. It is also investigating its credit card rival Visa.