The World Trade Organization has concluded that China is discriminating against U.S. credit card companies by denying them full access to the Chinese market.

The WTO Monday sided with U.S. claims that a Chinese company, China UnionPay, discriminated against foreign financial institutions by not allowing them legitimate access to the country's electronic-payment systems. The ruling can be appealed to the WTO's Appellate Body, but as it stands, would give such American companies as Visa, Mastercard and American Express new access to the vast Chinese market.

The WTO did not rule entirely in favor of the U.S., which first brought the case in 2010. But the Geneva-based agency concluded that Beijing maintains China UnionPay as a monopoly to handle local currency electronic payments. The WTO said China had acted “inconsistently” with international trade rules.

The ruling said China UnionPay acted improperly by requiring that all payment cards in China have the “UnionPay” logo and that all cash machines be equipped to accept them.

U.S. Trade Representative Ron Kirk said the ruling showed that “China's pervasive and discriminatory measures deny a level playing field” to the American companies.