A federal judge ruled on Thursday that American Express' rules for merchants violate antitrust laws and prevent retailers from encouraging consumers to use lower-cost cards.
U.S. District Judge Nicholas Garaufis deemed the company's non-discrimination provisions anti-competitive, saying they prevent the roughly 3.4 million merchants who accept its cards from "steering customers" to alternative credit card brands, such as Visa, MasterCard, and Discover.
The merchant's cost of accepting American Express—one of the three largest network services providers in the U.S.—tends to be greater than the cost of accepting other cards, according to the ruling.
The credit card issuer said it believes the decision was wrong and it plans to file an appeal.
"The court's ruling will not provide any benefit to consumers and will, in fact, harm competition by further entrenching the two dominant networks," the company told CNBC in a statement. "We continue to believe that the Department of Justice's arguments are flawed and believe we should prevail on appeal."
AmEx shares fell more than 2 percent after the 150 page verdict surfaced.
The decision is the latest blow to AmEx, which recently announced that it would cut more than 4,000 jobs over the next year as it battles to control costs.
Last week, AmEx said it would end its years-long partnership with Costco Wholesale after the two parties failed to agree on terms that "would have made economic sense" for the company and its shareholders. Costco accounted for a whopping 8 percent of AmEx's global billings and a tenth of cards outstanding, according to a Financial Times report.