More than 40,000 Asda workers have won a Supreme Court appeal case over a long-running pay dispute.
The ruling means shop floor staff can be compared to workers in distribution centres for the purposes of their equal pay claim.
GMB Union hailed the ruling on Friday a “massive victory” and said it would enter into discussions with the supermarket over what it said could be £500m in compensation.
The Court of Appeal, the UK’s highest court, ruled against Asda after shop workers, who are mostly women, brought equal pay claims on the grounds that warehouse workers, who are mostly men, unfairly earn more.
Wendy Arundale, who worked for Asda for 32 years, said: “I’m delighted that shop floor workers are one step closer to achieving equal pay.
“I loved my job, but knowing that male colleagues working in distribution centres were being paid more left a bitter taste in my mouth.
“It’s not much to ask to be paid an equal wage for work of equal value, and I’m glad that the supreme court reached the same conclusion as all the other courts.”
Lauren Lougheed, a partner at law firm Leigh Day, said: “We are delighted that our clients have cleared such a big hurdle in their fight for equal pay.
“Already an employment tribunal, the Employment Appeal Tribunal and the Court of Appeal ruled that these roles can be compared, and now the supreme court has come to the same conclusion.
“It’s our hope that Asda will now stop dragging its heels and pay their staff what they are worth.”
The ruling does not mean the workers have won their claim to equal pay but it does allow them to take further action, including going to further employment tribunal cases to determine if particular shop and distribution jobs are of comparable “value”.
Susan Harris, GMB legal director, said the ruling was a “massive victory” for the supermarket chain’s mostly female shop floor workforce.
She said: “Asda has wasted money on lawyers’ bills chasing a lost cause, losing appeal after appeal, while tens of thousands of retail workers remain out of pocket.
“We now call on Asda to sit down with us to reach agreement on the back pay owed to our members – which could run to hundreds of millions of pounds.”
Asda had argued that shop floor work was not comparable to that in the warehouses.
An Asda spokesman said: “This ruling relates to one stage of a complex case that is likely to take several years to reach a conclusion.
“We are defending these claims because the pay in our stores and distribution centres is the same for colleagues doing the same jobs regardless of their gender.
“Retail and distribution are very different sectors with their own distinct skill sets and pay rates.
“Asda has always paid colleagues the market rate in these sectors and we remain confident in our case.”