By Robin Sidel 

A U.S. judge ruled on Thursday that American Express Co.'s rules for merchants violate antitrust law, dealing a major setback to the card company that has been hit by a series of blows.

The case, which was heard by a judge last year, stems from a 2010 lawsuit in which the Justice Department contended that AmEx's rules for merchants inhibit competition and result in higher fees for consumers.

The rules at issue bar AmEx merchants from telling customers they prefer to accept one card over another.

U.S. Judge Nicolas Garaufis said in a 150-page verdict that AmEx's rules "constitute an unlawful restraint on trade."

AmEx has repeatedly defended its rules and said in financial filings that it could suffer a "material adverse effect on our business" if it loses the case.

On Thursday, AmEx said in a statement that it was disappointed by the decision and will appeal "because we believe the decision was wrong."

American Express shares dropped more than 2% in midmorning trading.

Among other things, AmEx prohibits merchants who accept its cards from steering customers toward other forms of electronics payment, such as credit cards that are processed by rivals Visa. Inc. or MasterCard Inc. or debit cards.

"The court's ruling will not provide any benefit to consumers and will, in fact, harm competition by further entrenching the two dominant networks," AmEx said in its statement.

The verdict was the latest blow for AmEx, which has been falling short of its revenue targets. The company last week announced that its 16-year-old partnership with Costco Wholesale Corp. would dissolve next year after the two sides were unable to reach agreement on terms for a new co-branded card.

AmEx Chief Executive Officer Kenneth Chenault testified in the antitrust trial, saying that the company's reputation is hurt when merchants steer customers away from using AmEx cards.

"The most damaging thing that can happen to the American Express brand is when our card members are discriminated against at the point of sale, " he said.

Write to Robin Sidel at robin.sidel@wsj.com

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