Supermarket giant Asda is facing legal action from up to 19,000 women over equal pay.
Law firm Leigh Day said the claims were launched after investigations revealed some store employees were being paid less than colleagues doing the same job in its distribution centres.
The pay difference between shop workers, who are mainly female, and warehouse workers, who are mainly men, could be as much as £4 an hour.
If the claimants are successful, they could be entitled to six years' worth of back pay in compensation.
Michael Newman, a discrimination and employment law expert at Leigh Day, warned the implications of these legal claims were "enormous for Asda and many other supermarkets in the UK".
The challenge is thought to be the largest challenge of its kind in the private sector and could act as a "test case" for other retailers.
An Asda spokesman said they would "robustly defend" the claims.