A federal judge in San Francisco on Tuesday granted class-action status for a sex discrimination lawsuit against Wal-Mart Stores Inc. that could affect as many as 1.6 million current and former female employees.

The suit alleges that Wal-Mart frequently pays women less than men for comparable jobs and bypasses women for key promotions.

Wal-mart will appeal. "We think it is wrong certainly. And so, we will just have to wait and see what an appeal court does," chairperson Rob Walton said.

The action, filed three years ago, has become the largest private civil rights case in U.S. history.

U.S. District Judge Martin Jenkins took nine months to decide that the lawsuit should include virtually all women who have worked at Wal-Mart's 3,500 stores in the U.S. since 1998.

Wal-Mart, which is the largest private employer in the U.S., tried unsuccessfully to limit the legal action to small class-action suits against individual stores.

Jenkins said the "plaintiffs present largely uncontested descriptive statistics which show that women working at Wal-Mart stores are paid less than men in every region, that pay disparities exist in most job categories, that the salary gap widens over time and that the higher one looks in the organization, the lower the percentage of women."

Joseph Sellers, one of the lawyers representing the women, told the Associated Press, "I think it's a terrific victory for the women who work at Wal-Mart who have laboured for years under working conditions where they have been told repeatedly they have been unsuitable for management and not suitable to make as much as men."

Wal-Mart spokesperson Mona Williams said the company is confident it does not discriminate against female employees.