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 in a trial that lasted seven weeks last summer, 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 government wasn't seeking monetary damages in the case, but instead was trying to force AmEx to drop its restrictions.

U.S. Judge Nicholas Garaufis said in a 150-page verdict that AmEx's rules "constitute an unlawful restraint on trade." The case was heard in federal court in Brooklyn, N.Y.

One of the key rules at issue bars merchants who accept AmEx cards from encouraging customers to use another card other than AmEx.

AmEx has repeatedly defended its rules and said in financial filings that it could suffer a material adverse effect on its business if it lost the case. AmEx uses the fees that it charges to merchants to fund its well-known rewards program and provide other perks to its cardholders.

The ruling doesn't mean that AmEx must drop its rules immediately. The judge has asked both sides to submit a proposed remedy to the situation because "the parties themselves are likely best equipped to determine how American Express' merchant regulations might be rewritten so as to satisfy American Express' interests" and comply with antitrust rules.

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 about 2% in midmorning trading and are off 16% so far this year.

The ruling comes at a tough time for AmEx, which has been falling short of its revenue targets. To counter that, the company is trying to expand its acceptance among small merchants and is also broadening into prepaid debit cards for consumers who normally wouldn't qualify for a traditional AmEx card.

Last week, AmEx 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 is currently the only credit card that is accepted at Costco stores, although the chain also accepts debit cards.

Meanwhile, rivals Visa Inc. and MasterCard Inc. had similar merchant rules in place for years, but scrapped them in 2010 just before the justice department sued AmEx. Visa and MasterCard agreed to allow merchants to offer discounts, rebates and other incentives to get customers to use cards with lower merchant fees, such as "plain vanilla" plastic that doesn't have any rewards programs or annual fees.

AmEx, however, refused to settle the case, saying it isn't big enough to be an anticompetitive presence in the industry. There were 53.6 million AmEx cards in circulation in 2013 compared with 254.1 million U.S.-issued cards from Visa and 178.3 million cards from MasterCard, according to court documents.

Among other things, merchants who accept AmEx cards aren't permitted to steer customers to other cards, such as those branded by Visa and MasterCard.

"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.

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.

In his decision, Judge Garaufis said that he had urged both sides to settle the case and that "the court does not come to its decision in this case eagerly or easily."

"The court recognizes that it does not possess the experience or expertise necessary to advise, much less dictate to, the firms in this industry how they must conduct their affairs as going concerns," he wrote.

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

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