The US Supreme Court is set to rule on whether up to 1.5 million women, who are current and former employees of Walmart, can sue the company for sexual discrimination. If allowed to proceed, it would be the biggest civil rights class action in US history and could cost the retailer billions of dollars. The women claim the company paid male colleagues more for the same work and that they were discriminated against in terms of salary and promotions.

The case began in 2000 when former Walmart employee Betty Dukes filed a sex discrimination suit claiming she had been denied the training she needed to receive promotion. Another six women later joined the action, with more Walmart workers following.

Representing Dukes, civil rights lawyer Brad Seligman claimed Walmart systematically discriminated against female employees, who were underrepresented in management positions and were paid less than male colleagues.

The lawsuit claims that women account for only a third of what Walmart considers management.

Certification of the class action was approved by a federal appeals court, but Walmart asked the Supreme Court to overturn the decision, claiming management policies were varied, from store to store.

The firm denies wrongdoing and argues that the accusations are too numerous and too diverse to be tried en masse.

Around twenty major firms including General Electric and Microsoft have filed papers with the court in support of Walmart.

If the Supreme Court allow the class action to go ahead, legal experts say it could redefine discrimination law in the US.