WASHINGTON (AFP) – The US Supreme Court on Tuesday will hear a potentially landmark labor case on whether 1.5 million women employed by retail giant Wal-Mart can file a discrimination lawsuit as one unified group.

Female workers at the company, which is America's largest employer, claim that over the years, they systematically received lower pay than their male counterparts and were passed over for promotions, and are asking the US high court for the right to pursue legal action against the company as a group.

Lawyers representing the plaintiffs note that women at Wal-Mart make up about two-thirds of the hourly workers, but are only a fraction of store managers.

They also point out that in nearly every job category, women earned less than men, even though most had logged more years with the company than their male counterparts.

The women are seeking back wages they believe they are owed and punitive damages against Wal-Mart, as well as a change in its pay practices, which would affect both current and past company employees.

"Plaintiffs presented evidence that Wal-Mart managers undervalued women and espoused outdated stereotypes about their roles in the workplace," said Ariela Migdal, staff attorney with the Women's Rights Project at the American Civil Liberties, which has filed a friend of the court brief in the case.

"The women allege that Wal-Mart's nationwide practice of leaving pay and promotion decisions up to local managers allowed the stereotypes to limit their opportunities. Because of this, the women should be able to pursue their class action," said Migdal.

If the court allows the women to sue en bloc, the case would constitute the largest job-discrimination case in US history, with tens of billions of dollars at stake.

In seeking to block the case, Wal-Mart warned in August last year of the enormity of the class action, describing it as "larger than the active-duty personnel in the Army, Navy, Air Force, Marines, and Coast Guard combined."

Even more significantly, the ruling is expected to set a precedent for future labor discrimination lawsuits, opening the door for a possible flood of broadly defined "class action" cases on behalf of women, minorities and people with disabilities.

Legal observers say it is unlikely to rule in favor of the women, noting the US high court's pro-business, free market bias.

Wal-Mart has fought fiercely to defeat class action status for the suit, which was filed a decade ago by seven female Wal-Mart workers.

The often liberal-leaning Ninth Circuit Court in San Francisco agreed by a 2-1 vote in 2007 and again in a narrow 6-5 decision in April of last year, deciding both times in favor of the women employees.

Each time, Wal-Mart appealed the ruling, and the case now goes to the highest US court, which is expected to issue its ruling at the end of June.

If the company loses, it faces the risk of legal action from the ensemble of female workers employed at Wal-Mart since 1998

On the other hand, a win for Wal-Mart could be a blow to nationwide job bias suits.

For its part, Wal-Mart maintains that it is impossible to assert a case of discrimination based on company employment figures, and say there is no pay difference between men and women at the vast majority of its outlets.

Instances where those differences exist are made on merit, not because of a discriminatory company-wide policy, Wal-Mart insists.