Supermarket giant Asda is paying its female-dominated shopfloor staff less than its male-run distribution workforce despite the similarities in job function, the GMB union claimed this week.

Test cases against Asda are looming as the union turns the spotlight on the retail industry as part of a wider campaign for equal pay for women, which if successful could result in million of pounds paid in extra wages.

The aim is to test whether Asda's largely female shopper workforce can claim higher pay akin to that of its distribution workforce.

"There is a case to answer as shopfloor workers are basically doing the same job as the guys in the warehouses, but in reverse," said one union source. "Distribution operatives are picking stock for delivery, while shopfloor workers earning up to £3 less per hour are putting that same stock on to shelves."

The Asda cases will have to go through an internal grievance process before going to an employment tribunal. A tribunal would then appoint an expert to assess whether the jobs were different.

"It would assess whether there were genuine reasons for different terms, such as distribution centres being in more remote locations than stores," said Pam Loch of commercial employment experts Loch Associates.

The test cases are the first the GMB has undertaken in the retail sector, though it has lodged 25,000 cases for local council workers.

Asda insisted it was in favour of equal pay. "We have been contacted by the GMB about the general issue of equal pay. We have asked them to provide us with more details so we can give it proper consideration," it added.

The precedent for equal gender pay claims among workers in different jobs but within the same company was set in the public sector in 1997, following a landmark case in which 1,500 Cleveland dinner ladies won a £5m payout.

Ten years after the Cleveland case, council workers across the country are still successfully pursuing equal pay claims, which have cost councils millions of pounds, according to Loch.