The landmark antitrust suit against Visa U.S.A. and MasterCard International by the retailing industry has been granted class-action status.

In a decision filed Tuesday evening at U.S. District Court for the Eastern District of New York, Judge John Gleeson concluded that "all persons and business entities who have accepted Visa and/or MasterCard credit cards and therefore have been required to accept VisaCheck and/or MasterMoney debit cards under the challenged tying arrangements" should be grouped as one class.

The plaintiffs in the case, led by Wal-Mart Stores Inc., Sears Roebuck & Co., and other major retailers, estimate that the class includes about four million merchants.

The lawsuit, which dates from 1996, alleges that the tying of debit and credit card acceptance violates antitrust law.

The ruling is a setback for the card associations, which stand to lose billions of dollars in damages. The retailers are seeking an estimated $8.1 billion in preliminary damages, and if the "honor all cards" rule is not lifted, damages could reach $63 billion over the next 10 years before trebling, according to Lloyd Constantine, lead attorney for the retailers.

Both MasterCard and Visa said they would appeal the decision within 10 days and said they are confident the case will ultimately lose class-action status.

"We're extremely encouraged because the judge felt restrained in this order," said Kelly Presta, a vice president at Visa. "The judge is encouraging appeal of this, and we're very optimistic about that."

In a somewhat unusual move, Judge Gleeson did recommend that the Second U.S. Circuit Court of Appeals, which would hear any appeal of his decision, "take an interlocutory appeal" of his order.

"This litigation poses enormous financial risks for the defendants, risks that are obviously increased drastically by certification of the class," Judge Gleeson wrote in his ruling. "Moreover, this certification motion raises substantial and novel questions involving the standards a district court should apply in evaluating a class motion and the interaction of those standards with antitrust principles."

However, Mr. Constantine said the case is very unlikely to lose its class-action status.

"Without the judge's recommendation the chances were one in a thousand," he said. "Now they are one in one hundred. Even if the Second Circuit entertains an appeal, we're very confident that the Circuit will agree with the district court, and the schedule will be maintained."

Trial is set for Nov. 27.

Noah Hanft, head U.S. counsel for MasterCard, said that even if the case remains a class action he does not believe the plaintiffs will win.

"The decision does not relate to the merits of the case," he said, "and we continue to be confident that our rules, which serve to protect consumers and ensure that their MasterCard cards are accepted by all MasterCard merchants, will be upheld and that this lawsuit is groundless."

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