Supermarket giant Asda has lost an appeal in the latest development in a long-running legal dispute with staff over equal pay.
The decision means that lower paid shop staff, who are mostly women, can compare themselves with higher paid warehouse workers, who are mostly men.
Asda said it was "disappointed" with the decision and added it remained confident in its case.
A ruling over whether the work is of equal value is likely to be in May.
Leigh Day, which represented the staff, said the judgement was a "major step forward in the fair pay battle".
Asda said: "We are obviously disappointed with the decision, which relates to a preliminary issue of whether jobs in different parts of the business can be compared."
It said it had brought the appeal "because it involved complex legal issues which have never been fully tested in the private sector and we will continue to ensure this case is given the legal scrutiny it deserves".
The Employment Tribunal first ruled against Asda in October 2016. It said shop workers, who mainly work at check-outs or stacking shelves, could compare themselves with staff who work at warehouses.
Asda then appealed against this decision on 10 different grounds.
In August 2017, the Employment Appeal Tribunal ruled all points of their appeal unsuccessful. Asda then took its case to the Court of Appeal.
Following Thursday's ruling, the Court of Appeal denied Asda the right to appeal. However, the BBC understands the supermarket chain intends to apply to the Supreme Court to appeal against the ruling there.
There are three key stages in an equal pay case
- Are the jobs comparable?
- If the jobs are comparable, are they of equal value?
- If they are of equal value, is there a reason why the roles should not be paid equally?
Leigh Day represents more 30,000 shop floor staff from the big four supermarkets - Asda, Sainsbury's, Tesco and Morrisons - in similar cases.
The legal firm said if the four supermarkets lost their cases and were ordered to pay all eligible staff, the cost could be more than £8bn. However, that would only be if all 500,000 store workers made a claim.
The GMB union, which represents some Asda workers, welcomed what it described as Thursday's "landmark" judgment.
General secretary Tim Roache, said: "We know we're not all the way there, there are more hurdles to jump in this process and as always we remain ready to negotiate should Asda want to get round the table."
Asda said: "Our hourly rates of pay in stores are the same for female and male colleagues and this is equally true in our depots.
"Pay rates in stores differ from pay rates in distribution centres because the demands of the jobs in stores and the jobs in distribution centres are very different; they operate in different market sectors and we pay the market rate in those sectors regardless of gender."