Workers from Asda have held demonstrations to mark the start of an equal pay claim involving more than 60,000 staff.
The GMB union said the supermarket chain’s predominantly female retail workforce is paid up to £3.74 an hour less than the mainly male staff in warehouse roles.
Staff held a protest in Manchester earlier, where the case has been brought before an employment tribunal, to call for pay parity.
Asda said it respected the right of staff to bring the case but “strongly rejects” claims that pay rates are influenced by gender.
The GMB has described the case as the biggest ever in the private sector, with the tribunal hearing expected to last three months.
Those behind the claim, brought forward by law firm Leigh Day, argue that retail work is of equal value to the company as warehouse work.
A spokesman for the law firm said if the claim was successful staff in the underpaid roles would be able to claim six years’ worth of back pay as compensation.
It is the second stage of a long-running case which began in 2014.
Nadine Houghton, GMB national officer, said the outcome of the hearing would “call time on the retailers undervaluing their predominantly women shop floor workers”.
She said members were challenging a sector that “has been built on the structural undervaluing of women’s work”.
A spokeswoman from Asda said there was a range of different job roles working in retail and warehouse positions.
“We continue to defend these claims because retail and distribution are two different industry sectors that have their own distinct skill sets and pay structures,” she added.
The case follows the end of a similar equal pay legal battle involving thousands of Next workers, which found retail staff should not have been paid at lower rates than warehouse staff.