Co-op shop floor workers have won a key legal argument in a battle to secure equal pay with warehouse staff.
More than 1,600 mostly female supermarket workers have been fighting for pay parity with mostly male staff at distribution centres, who are paid up to £3 an hour more.
Co-op has conceded a "comparability concession", a step towards recognising the different roles are of equal value.
But it said its workers were "fairly" paid and the battle was far from over.
It comes amid similar equal pay battles at rival supermarkets Tesco, Asda, Sainsbury's and Morrisons.
Co-op made its concession as a part of an ongoing pay tribunal. Tom Hewitt of solicitors Leigh Day, which is representing the workers, said Co-op shop floor workers had now "cleared the first hurdle in their claims for equal pay".
"We hope that Co-op recognises that they can no longer deny that the work store workers do is of equal value to that of their distribution centre colleagues," he said.
The claim began after the mostly female shop floor employees found they were being paid less than men in Co-op's warehouses.
This made them feel they were "underpaid for the same effort", Leigh Day said.
The law firm said Co-op's concession was the first stage in a three-step process that could see the workers reclaiming thousands of pounds of missed back pay.
The retailer will now have to show that the roles are not of equal value or that there is a genuine reason for the pay difference which is not based on gender.
A Co-op spokesman said: "Our colleagues play an important role in feeding the nation and it's central to the Co-op's values that we pay them fairly for the work that they do in supporting communities.
"We believe that we pay our colleagues fairly for the roles that they do, and so will continue to defend these claims."
£8bn of back pay claims
It is the latest of a number of equal pay fights that could end up costing grocery chains up to an estimated £8bn in back pay claims.
In September 2021, an employment tribunal ruled that the roles of Morrisons' store workers could be compared to their colleagues in distribution centres.
The case has proceeded to further hearings examining whether store worker and distribution roles are of equal value.
In March 2021, a landmark judgment was handed down by the Supreme Court which confirmed that Asda's shop floor workers could compare their roles to those of colleagues in distribution centres for the purposes of equal pay.
However, the judge stressed the ruling did not mean the 44,000 claimants had won the right to equal pay, only that they were free to take further legal action.
Meanwhile, in June 2021 thousands of Tesco shop floor workers celebrated a European court ruling that an EU law could be relied on in making equal pay claims against their employer. It argues that a worker can be compared with somebody working in a different establishment if a "single source" has the power to correct the difference in pay.
The Tesco workers, mostly women, had argued that they failed to receive equal pay for work of equal value with colleagues in its distribution centres, who are mostly men.
Following the ruling Tesco said: "These claims are extremely complex and will take many years to reach a conclusion. We continue to strongly defend these claims."
Lawyers say the litigation could run on for years.