Supermarket giant Asda is being urged to solve a "wide-ranging" industry problem of equal pay after a ruling on jobs of equal value.
The Employment Appeal Tribunal decided that jobs in Asda stores were comparable to those in the firm's distribution centre.
The GMB union said it wanted to find a "sensible solution" to its demand that female store workers should be paid the same as men.
Law firm Leigh Day said the ruling meant that Asda has been unsuccessful in attempting to stop more than 10,000 equal pay claims from proceeding.
Chris Benson, head of the employment and discrimination department at Leigh Day, said: "Asda continues to appeal every point available to them, rather than focusing on paying men in the distribution centres and women in the stores equally, but judges at every level have been adamant that the claims can continue.
"After yet another defeat, we hope that Asda take this opportunity to reflect on the merits of the claims, and concentrate on why they pay men more than women for jobs of equal value, rather than trying to stop the claims going ahead at all."
Tim Roache, GMB general secretary, said: "GMB look forward to Asda management sitting down and finding a sensible negotiated solution to recognising that our female members in stores should be paid and valued as equal to the men.
"Instead of wasting money on litigation, we ask Asda to be a market leader in solving this wide-ranging industry problem."
If the shop workers are successful in their claims, Asda are likely to have to review the pay of tens of thousands of staff who may have been paid unlawfully, said Leigh Day.
An Asda spokesman said: "We are disappointed with this appeal ruling which relates to a technical preliminary issue of whether jobs in different parts of the business can be compared.
"The Employment Appeal Tribunal have given us permission to appeal against this judgment, to the Court of Appeal.
"We continue to strongly dispute the claims being made against us. The employment tribunal has yet to consider whether the jobs are of equal value in terms of their demands and, if some jobs are, only then will the tribunal move on to consider the reasons for the differentials, including the existence of different market rates in different industry sectors.
"At Asda hourly-paid colleagues doing the same job in the same location are paid the same. Men and women doing the same job in our retail stores are paid the same. Men and women doing the same job in our distribution centres are paid the same.
"Pay rates in stores differ from pay rates in distribution centres for legitimate reasons, including the different market rates for different jobs in different sectors."
A lawyer said the ruling could have "far-reaching consequences".
"This decision by the Employment Appeal Tribunal, which will bind lower Tribunals, may have far reaching consequences in businesses where female employees are earning less than their male counterparts for doing similar work of equal value in different departments," said J eremy Coy, who is based at law firm Russell-Cooke.
"It appears, however, that Asda intends to appeal so we may have to await a further judgment from the Court of Appeal before this area of law becomes clearer."