The lawsuit alleges women staff get a bad deal from Wal-Mart

The judge's decision means the case now involves up to 1.6 million women who have worked for Wal-Mart since 1998.

It is the biggest civil rights case against a private employer in US legal history, the plaintiffs' lawyers say.

Wal-Mart, which is the world's biggest retailer, has not yet made any comment on the legal setback.

The original lawsuit was filed in June 2001 by six women who either worked for Wal-Mart or had done so in the past.

Pay and promotion

It alleged that Wal-Mart systematically paid women less than men and passed them over for promotion.

The suit covers staff at US stores

San Francisco-based judge Martin Jenkins dismissed Wal-Mart's argument that a class action lawsuit would be too unwieldy.

"Up till now, Wal-Mart has never faced a trial like this," said Brad Seligman, the lead counsel for the women.

"Lawsuits by individual women had no more effect than a pinprick. Now, however, the playing field has been levelled. Wal-Mart will face the combined power of 1.6 million women in court," said Mr Seligman in a statement on the Impact Fund website, a campaign group backing the women.

It says Judge Jenkins described the lawsuit as "historic in nature" in deciding to certify it as a class action.

Wal-Mart has 1.3 million staff and sources goods for its gigantic store chains, Wal-Mart and Sam's Clubs, from 23 countries.

Fair comparison?

Wal-Mart had argued that a class action was inappropriate because of its size, and because promotions were decided by local managers.

The plaintiffs lawyers opposed this on the grounds that Wal-Mart is a tightly knit and highly regulated company with a strong corporate culture. Its 3,500 US stores are more or less identical, they argued.

Wal-Mart has 10 days to ask the US Ninth Circuit Court of Appeals to review Judge Jenkins's decision, the Wall Street Journal reported.

If the appeal court upholds the judge's decision to allow a class action, it could still be several months before the hearing begins.

"Let's keep in mind that today's ruling has absolutely nothing to do with the merits of the case," the Wall Street Journal quoted Wal-Mart spokeswoman Mona Williams as saying.

The class action lawsuit will only apply to women employed by Wal-Mart in the US since 26 December 1998.