Image caption Betty Dukes, left, and plaintiff Christine Kwapnoski said they were discriminated against

The US Supreme Court has been hearing evidence on whether the largest sex discrimination lawsuit in American history should go ahead.

A group of women is suing the world's biggest retailer, Wal-Mart, claiming they were held back because of their gender.

They want to bring a class action suit on behalf of more than a million women.

Wal-Mart denies the allegations and says it promotes women employees and pays them well.

The group bringing the lawsuit says Wal-Mart systematically discriminated against women in stores across America.

But lawyer Theodore Boutrous Jr, for Wal-Mart, said on Tuesday the class action nature of the legal action deprived the company of legal rights as it was forced to defend the treatment of women regardless of the jobs they held or their place in the Wal-Mart chain.

Analysis This is the first step in what those bringing the case hope could become a landmark legal moment. What started out as a grievance over pay and promotion among a handful of women could turn into the largest sex discrimination lawsuit in America. The first claims of women being passed up for promotion at Walmart stores surfaced 11 years ago, but the six women pursuing this case do not want their grievances to be heard separately. Instead, they are pushing for the case to be heard as a class action - to include all female store employees since 1998 - because they claim there is a wider culture of sex discrimination in the company. It is up to America's highest court to decide whether this is how the case should proceed. Walmart denies the charges, but if a class action goes ahead, the company faces the prospect of a billion-dollar payout, as well as a dent to its reputation.

"There is absolutely no way there can be a fair process here," Mr Boutrous said.

Justice Ruth Bader Ginsburg said the issue at stake at this early stage in the process was not proving discrimination, but showing enough evidence to go forward.

"We're talking about getting a foot in the door," Justice Ginsburg said

Meanwhile, Justice Anthony Kennedy remarked that he was unsure "what the unlawful policy is" that Wal-Mart engaged in.

A decision on whether the class action suit should go forward is expected by summer.

'Lost pay'

Christine Kwapnowski, one of six women named as plaintiffs in the lawsuit, says she was passed over for pay and promotion in favour of male colleagues.

"I asked what I needed to do to get promoted and my manager said I should 'doll up and blow the cobwebs off my make-up,'" she told the BBC.

The six plaintiffs bringing the lawsuit are making their claim under the 1964 Civil Rights Act, arguing "the policies and practices underlying this discriminatory treatment are consistent throughout Wal-Mart".

The women, who are seeking lost pay and damages, want the US Supreme Court to allow the case to proceed as a class action lawsuit against the company.

A class action suit would cover any woman who has worked for, or works for, one of more than 3,400 Wal-Mart stores in the US since December 1998.

Image caption A group of women demonstrated outside the court on Tuesday, chanting for "fair pay now"

Two lower courts have allowed the suit to proceed as a class action; Wal-Mart appealed to the Supreme Court. A decision is not expected until June.

Wal-Mart denies the claims, and notes the company has won awards for its women-friendly working practices.

"Wal-Mart has had - for many years - strong policies against discrimination and these policies are there to ensure women are promoted and paid well," company spokesman Greg Rossiter said.

After the hearing Ms Kwapnoski said she was confident about the outcome.

"Ten years has been how long this has been going on," she said. "And it's a fight worth fighting and we'll keep fighting it until the end."