Workers from supermarket giant Asda will stage demonstrations on Monday to mark the start of an equal pay claim.
The GMB said the case involves more than 60,000 workers, calling it the biggest ever in the private sector.
Workers will demonstrate outside the TUC Congress in Brighton and in Manchester where the case will be held.
The GMB said the case is expected to last three months.
The result of this hearing will call time on the retailers undervaluing their predominantly women shop floor workers
The union is arguing that the predominantly female retail workforce is paid up to £3.74 an hour less than the mainly male warehouse workforce.
Claimants will say that retail work is of equal value to the company as warehouse work.
It is the second stage of a long-running case.
Nadine Houghton, GMB national officer, said: “Asda workers are making history. The result of this hearing will call time on the retailers undervaluing their predominantly women shop floor workers.
“The entire retail sector has been built on the structural undervaluing of women’s work but GMB members are changing this.”
An Asda spokesperson said: “We fully respect the right of current and former colleagues to bring this case, however, we strongly reject any claim that Asda’s pay rates are influenced by gender.
“There are numerous different jobs within retail and within warehouses. 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.”
