White House spokesman Jay Carney said ending pay discrimination was a key priority for President Barack Obama but that the U.S. government was not a party to the lawsuit.

"Our lawyers are studying the decision right now to determine what effects it might have," Carney said.

The Obama administration did not take a position in the top court case, although the federal government's Equal Employment Opportunity Commission previously supported the workers.

MOST IMPORTANT CLASS-ACTION RULING SINCE 1960S

Christopher Landau, an attorney at the Chicago-based Kirkland & Ellis law firm, called it the most important class-action decision since the 1960s. "The days of 'drive-by' class certification are over," he said.

Justice Antonin Scalia concluded for the court majority that the Wal-Mart class was not properly certified.

"Because respondents wish to sue about literally millions of employment decisions at once, they need some glue holding the alleged reasons for all those decisions together," he said.

The court's four other conservatives joined all of Scalia's ruling. The court's four liberals joined part of it but dissented in another part.

Large class-action lawsuits make it easier for big groups of plaintiffs to sue corporations and they have led to huge payouts by tobacco, oil and food companies.

Companies such as Wal-Mart have sought to limit such lawsuits to individual or small groups of plaintiffs. The Supreme Court, with a conservative majority that often ruled for businesses, has rejected huge class-action lawsuits.

The Supreme Court case is Wal-Mart Stores Inc v. Betty Dukes, No. 10-277.

(Additional reporting by Jessica Wohl in Chicago, Moira Herbst in New York and Jeremy Pelofsky and Matt Spetalnick; Editing by Doina Chiacu and Bill Trott)