Washington: The US Supreme Court unanimously rejected a class action suit against retail giant Wal-Mart by 1.5 million female workers who claimed they had been systematically discriminated on the basis of gender.

In the most closely watched decision of the year, the top US court ruled that Wal-Mart had no overarching employment policy so that all women may not suffer discrimination for the same reason.

Had the court decided in favour of the plaintiffs, the largest class action suit in history could have cost the world's biggest retailer tens of billions of dollars in back pay and punitive damages.

By rejecting the suit, nine justices limit the recourse available to the women, who claim to have been paid far less and benefited from fewer promotions than their male counterparts.

"The basic theory of their case is that a strong and uniform 'corporate culture' permits bias against women to infect, perhaps subconsciously, the discretionary decision-making of each one of Wal-Mart's thousands of managers -- thereby making every woman of the company victim of one common discriminatory practice," the justices said in their unanimous decision.

But they said the only convincing evidence that the plaintiffs had been able to present was that Wal-Mart's policy was to allow local supervisors discretion over employment matters.