The court upheld an earlier court ruling that means shop floor staff at Asda (mostly women) can be compared to workers in the distribution centre (mostly men) for the purposes of their equal pay claim.

The ruling is the fourth time Asda has lost a court battle on this issue.

However, the judge stressed the ruling did not mean the 44,000 claimants had won the right to equal pay – but they are now free to take further action.

Asda store workers argued they were paid less because most store workers are women, while most distribution depot staff are men. Depot staff were typically paid between £1.50 and £3 an hour more than store workers.

GMB Union says the ruling could lead to a potential compensation claim totalling £500m. It hailed the court’s decision as a “massive victory”.

In January 2019, the Court of Appeal ruled that GMB members could compare themselves in this way – upholding the rulings made by an employment tribunal in 2016 and the Employment Appeal Tribunal in 2017.

GMB has enlisted law firm Leigh Day to work the case on behalf of GMB members. There are almost 40,000 claimants involved.

Susan Harris, GMB legal director, said: “This is amazing news and a massive victory for Asda’s predominantly women shop floor workforce. We are proud to have supported our members in this litigation and helped them in their fight for pay justice.

“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.”

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.”

But an Asda spokesman told the BBC: “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.”