Washington State's highest court on Thursday let a class-action lawsuit alleging overcharging by Cingular Wireless go forward.

The state Supreme Court said a clause in Cingular's contract that prevented subscribers from starting class-action suits was unfair and sent the case back to the trial court in King County where it began.

The suit was filed in 2004 on behalf of a small number of Washington customers of Cingular Wireless, now part of AT&T Inc. It said the carrier advertised free roaming in areas covered by AT&T Wireless, which was then a separate company, but hit the customers with roaming charges anyway. It also said customers were improperly charged for long distance on some calls, said Douglas Dunham, an attorney representing the plaintiffs. They alleged Cingular had overcharged customers between $1 and $40 per month.

The plaintiffs' attempt to start a class-action suit was blocked by a clause in the Cingular customer contract, which said disgruntled subscribers couldn't form or join classes to sue the carrier. As a result, they didn't even get to the stage of finding out how many customers in Washington or nationally may have been affected, Dunham said. Now that they have won on appeal, the plaintiffs will pursue a class action, he said.

AT&T is studying the ruling, spokesman Walt Sharp said. The carrier could appeal the decision to the U.S. Supreme Court, according to F. Paul Bland, staff attorney at Public Justice, a public-interest law firm in Washington, D.C., that helped argue the plaintiffs' case.

In cases involving small amounts of money, consumers need to be able to pursue class actions, Bland said.

"Cingular was trying to write a contract term that would make it impossible for their customers to vindicate their legal rights ... if Cingular cheated them out of small sums of money," Bland said. "Even though Cingular broke its contract and engaged in a bait-and-switch that netted it millions of dollars, almost no individuals would ever be able to take their claims forward and get justice."

The decision will make it harder for other companies, such as credit card issuers, to introduce similar bans on class-action suits in Washington, Dunham predicted. In the Cingular case, it will probably take several months to get the class of plaintiffs certified and to gauge damages, he said. The case might not go to trial for two years, he added.