SAN FRANCISCO » The California Supreme Court has ruled that merchants can no longer ask for the ZIP codes of customers who make purchases with credit cards because such requests violate a state consumer protection law.

The high court’s unanimous decision, which says a ZIP code can be used as "personal identification information," overturned two lower court decisions tossing out the lawsuit. It delivered retailers in California a setback that an attorney for one national chain said would likely lead to additional lawsuits.

The decision last week came in a lawsuit filed against Williams-Sonoma Inc., whose clerk asked Jessica Pineda for her ZIP code several years ago. Pineda sued the home retailer in June 2008, saying it violated the credit card law and her privacy.

Williams-Sonoma and other merchants said they ask for ZIP codes, in part, as a security precaution.

Supreme Court Justice Carlos Moreno said the ZIP code is part of a customer’s address, which California law categorizes as off-limits. Gene Stonebarger, Pineda’s attorney, said gas stations that require ZIP codes be entered at the pump are exempt because they don’t record the transaction.