The lawsuit, originally filed on behalf of six women, becomes the nation's largest civil rights class action. Women employed at any time since December 1998 may join the suit against Wal-Mart, the largest U.S. private employer, with 1.2 million workers.

Workplace and legal experts said the decision sent a sobering message to corporate America, particularly retailers, which have a sizable female work force and have faced a series of sex discrimination lawsuits in recent years.

Wal-Mart, known for a take-no-prisoners stance in legal and labor relations battles, said it would appeal the decision. The Bentonville, Ark.-based company had sought to limit the scope of the lawsuit and denied a pattern of discrimination.

No trial date has been set.

"Prior to [Tuesday's] rulings, Wal-Mart had zero incentive to settle this case," said Perry Binder, legal studies professor at Georgia State University's Robinson College of Business. "However, now the plaintiffs can fight on a unified front and have leverage to force the issue."

Wal-Mart faces "a major undertaking" now that the case is a class action, said Gary Thomas, an Atlanta-based partner in the Fisher & Phillips law firm. He specializes in defending corporate management in employment discrimination cases. "I'm sure Wal-Mart will put forth a defense and defend the case on its merit."

Wal-Mart shares closed Tuesday at $54.06, down 87 cents.

Beyond the prospect of a long, costly court fight, Wal-Mart could be nursing a public relations black eye as it is undergoing growing pains.

Wal-Mart has often encountered community activists with a not-in-my-back-yard mentality. Now it faces some contentious rounds with workers who say their gender precludes them from advancing.

"This has been . . . building for a while," said Les Hough, director of Georgia State University's Center for the Workplace. "As their size has increased, their vulnerability has increased, and their growing pains, that is, their ability to adapt to their size, has been more limited."

Already the case has generated more than 200 depositions and a million pages of documents. One expert estimated if the case went to trial, damages could soar into the billions of dollars.

"If there was a settlement, it likely could dwarf the settlement in the Home Depot case," Binder said of the 1997 sex discrimination suit in which the Atlanta-based home improvement chain paid $104 million, without admitting wrongdoing, to 25,000 female employees.

As Wal-Mart grapples with image and employment policy issues, plaintiffs' lawyers and advocates call the ruling by U.S. District Judge Martin Jenkins a big win.

"The evidence points to widespread corporate practices that deny the civil rights of hundreds of thousands of women every day," said Carolyn Sapp, spokeswoman for a group known as Wal-Mart Versus Women.

Sapp said that although women make up 70 percent of Wal-Mart's work force, fewer than one-third of store managers are women.

She also said gender pay gaps widen further up the ladder. Full-time hourly female workers are paid 6.2 percent less than their male counterparts, and female senior vice presidents make half of what men in the same position are paid.

The Associated Press contributed to this article.