WASHINGTON — The U.S. Supreme Court will consider throwing out a massive lawsuit that claims Wal-Mart pays women less than men and promotes women less frequently.

The justices stepped into a case Monday that could involve 500,000 to 1.5 million women who work or once worked at the world's largest private employer. Wal-Mart Stores Inc. calls it the largest employment class action in history.

Wal-Mart is appealing a ruling by the 9th U.S. Circuit Court of Appeals in San Francisco that the class-action lawsuit could go to trial, with potentially billions of dollars in legal damages at stake.

Wal-Mart, based in Bentonville, Ark., and the many business interests backing its appeal, say allowing the large number of claims to go forward would set off an avalanche of similar class-action lawsuits in California and the other Western states overseen by the 9th Circuit.

But the lawyers representing the women who are suing Wal-Mart say there have been only eight such suits nationwide -- and none within the 9th Circuit -- since the first appeals court ruling in favour of the women nearly four years ago.

"This threatened landslide of class-action litigation has not materialized," the lawyers said in legal papers filed with the Supreme Court.

Wal-Mart employs 1.4 million people in the United States and 2.1 million workers in 8,000 stores worldwide. The company said the women should not be allowed to join together in the lawsuit because each outlet operates as an independent business. Since it doesn't have a companywide policy of discrimination, Wal-Mart argued that women alleging gender bias should file individual lawsuits against individual stores.

The plaintiffs contend that the company was aware that it lagged behind other employers in terms of opportunities for women and that Wal-Mart imposes uniform rules and tight controls over its stores.

The lawsuit was first filed by six women in federal court in 2001. The 9th Circuit has three times ruled that the case could proceed as a class action.

In its latest decision, in April, the appeals court voted 6-5 in favour of the plaintiffs. Judge Michael Daly Hawkins said that the number of women involved is large, but "mere size does not render a case unmanageable."

Judge Sandra Ikuta's blistering dissent said the women employees failed to present proof of widespread discrimination. Without such evidence, Ikuta said, "there is nothing to bind these purported 1.5 million claims together in a single action."

The case will be argued in the spring.

The case is Wal-Mart Stores v. Dukes, 10-277.