Supermarket giant Asda could face legal action from tens of thousands of mainly female employees who claim they have been underpaid.

Legal firm Leigh Day said they had been approached by 19,000 people, mainly women, after announcing it was taking action against the chain.

The firm said the legal challenge was launched after investigations revealed that employees in the Asda stores are being paid less than colleagues doing similar work in its distribution centres.

If the claimants are successful, they may be entitled to six years' worth of back pay to compensate for the difference in earnings, the firm said.

Michael Newman, a discrimination and employment law expert at Leigh Day, warned that the implications of these legal claims were "enormous for Asda and many other supermarkets in the UK".

He said: "In the supermarkets the check-out staff and shelf-stackers are mostly women. The people in the warehouses are pretty much all men. And, as a whole, the group that is mostly men gets paid more.

"Our investigations suggest that the jobs are pretty much the same, in that warehouse staff are responsible for taking items off shelves, putting them on pallets and loading them into lorries.

"In the supermarket, they do the reverse: taking the pallets off the lorries, unstacking them and putting the items on the shelves. Where the jobs are not similar, we still think they are of equal value."

A spokesman for Leigh Day said the legal challenge is believed to be the largest of its kind in the private sector.

He added: "Although there have been huge advancements in equal pay within the public sector, there is still a long way to go in the private sector. Compulsory audits are the only way to make sure the gender pay gap is being narrowed.

"Until now cases simply have not been brought, so you still get very segregated workplaces."

Chris Benson, a partner at Leigh Day, said the pay difference between shop and warehouse workers could be as much as £4 an hour - a "big difference" when some of the employees are earning £7 an hour.

He said: "The vast majority of claimants are still employed by Asda, they quite like the job but want to be paid equally going forward.

"The women are pretty aggrieved. Quite a lot of them feel that part of the reason it hasn't been resolved is because they are very loyal to Asda. They like the job and are committed to it, but want to be paid equally."

He warned the challenge could be a "test case" and said other major retailers could face similar actions.

He said the firm had so far lodged 1,500 claims at Manchester Employment Tribunal, and were considering thousands more.

An Asda spokesman said: "A firm of no-win, no-fee lawyers is hoping to challenge our award-winning reputation as an equal opportunities employer.

"We do not discriminate and are very proud of our record in this area which, if it comes to it, we will robustly defend."