NEW YORK ( TheStreet) -- A May 24 ruling against Mastercard ( MA) by the European Union's General Court isn't expected to impact earnings for the credit card company, though it may strengthen the hand of plaintiffs in a multi-billion dollar U.S. antitrust case pending against Mastercard, Visa ( V) and several giant banks.

On May 24, the EU General Court upheld a 2007 decision that Mastercard's cross-border transaction "interchange fees"--charged to retailers every time a card is swiped--were excessive.

While the ruling was "not a real surprise," it could impact future EU cases involving Mastercard, according to a May 24 report by Deutsche Bank analyst Brian Keane.

It would be more significant, however, if the EU decision impacted a civil case pending in the U.S. Eastern District of New York. That case, a lawsuit against MasterCard, Visa ( V), and 13 large banks, including Bank of America ( BAC)and Citigroup ( c) is expected to settle for a few billion dollars at the very least.

Critical to both cases is whether Mastercard's 2006 initial public offering protects it from accusations it illegally conspired with banks to set interchange fees. Prior to the offering, Mastercard was jointly owned by several large banks, including Bank of America, Citigroup, JPMorgan Chase ( JPM)and HSBC ( HBC).

The EU court ruled a conspiracy existed between Mastercard and several large European banks, including Banco Santander ( STD), Royal Bank of Scotland ( RBS) and HSBC ( HBC) despite Mastercard's IPO.

In the U.S. case, the defendants are asking the court to bar any antitrust claims against MasterCard after its 2006 IPO and against Visa after its 2008 IPO.

That motion by the defendants will be defeated, however, if the plaintiffs can show "there are some facts out there that demonstrate that, notwithstanding the IPOs Visa and MasterCard could still be deemed to be a conspiracy of banks," according to Matthew Cantor a partner in the law firm of Constantine Cannon, who has represented U.S. retailers in several matters as they fight against what they claim are excessively high bank interchange fees.

Cantor believes the EU decision provides the plaintiffs with additional ammunition as they try to negotiate a larger settlement with the banks and card companies.

The decision "would seem to indicate that there are factual issues in the very least as to whether or not MasterCard in the United States after its IPO is deemed to be a conspiracy of its members," Cantor says.

MasterCard spokesman Jim Issokson, however, argues "there is no connection" between the U.S. and European cases.

But Henry Polmer, an attorney who has represented merchants, card companies and card-issuing banks, argues the cases have a lot in common.

"One of the big questions they have in common is whether or not MasterCard was essentially acting as part of a conspiracy or in a coordinated effort with others and that together they had market power and they were essentially controlling and undermining competition," Polmer says. He argues the EU decision "doesn't bind the court in the United States, but will probably have some persuasive influence."

If MasterCard and Visa are deemed to be conspiracies even after their IPOs, it opens them up to billions more in potential damages, according to attorney Cantor.

-- Written by Dan Freed in New York.

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