A sex-discrimination lawsuit by female employees of Wal-Mart Stores Inc can proceed as a class-action case seeking back pay and other claims against the U.S. retailer, a U.S. appeals court ruled on Monday.

Wal-Mart, the world's largest retailer, had asked the Ninth Circuit Court of Appeals in San Francisco to undo class-action certification in what could be the largest sexual discrimination lawsuit in the nation's history.

The suit argues that female workers were paid less and received fewer promotions at Wal-Mart than male counterparts, and that the retailer's corporate structure fostered this gender discrimination.

The court said on Monday that it agreed with a lower court's ruling that female workers can bring claims for injunctive and declaratory relief and back pay through a class action lawsuit.

The Ninth Circuit sent back to the district court the claims for punitive damages.

The suit originated with a Wal-Mart worker named Betty Dukes who sued for sexual discrimination in 2001 with six other plaintiffs in a class-action lawsuit that extended the case to all women who had worked at the company since 1998.

A trial judge certified the case as a class-action matter in 2004.

(Reporting by Alexandria Sage; Editing by Tim Dobbyn)