Shopworkers at U.K. supermarket chain Asda have won the latest stage in their fight for equal pay in a ruling that could lead to a £500 million (US$688 million) compensation claim and could have big repercussions for the supermarket sector.

The supreme court has backed a 2016 employment tribunal decision that the supermarket’s retail staff, who are mostly women, can compare their work to those in warehouse distribution centers owned by the company, which is one of the U.K.'s 'Big Four' grocers.

More than 44,000 shop workers say they should be paid the same as the predominantly male staff who work in the chain’s depots, and who receive a higher wage of £1.50-£3 an hour more than the shop workers.

The GMB union, which is backing the case, called the supreme court ruling a “massive victory”.

Susan Harris, GMB’s legal director, 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," she said.

The win is the first major stage of the long-running court battle that has implications for workers in all the major supermarkets. The supreme court backed the 2016 employment tribunal ruling, which was also previously upheld by the court of appeal in 2019.

The outcome of the landmark case – the biggest-ever equal pay claim in the UK private sector – will have repercussions for about 8,000 workers at other supermarkets, including Tesco, Sainsbury’s, Co-op and Morrisons, who are also engaged in equal pay disputes with their employers.

Martin Chitty, employment partner at the law firm Gowling WLG, told The Guardian: "This is a further successful challenge to engrained gender /role based differentials in pay."

"The same challenges were brought successfully in the public sector 10 years ago where the equal value provided by women workers against historically male dominated roles was established. We now see this happening in retail – and other sectors need to be reviewing their pay structures and realigning voluntarily before claims are made."

The scale of any payout is likely to have implications for the group’s £6.5bn takeover by the billionaire Issa brothers, which is awaiting approval by competition regulators. Asda’s current owner, Walmart, which will retain a minority stake in Asda after the takeover, is reported to have agreed to pick up the tab for any compensation.