Asda has lost the first of three stages of its appeal against a ruling that store and warehouse staff deserve an equal hourly pay rate.
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 said it remained confident in its case.
A ruling over whether the work is of equal value is likely to be in May.
In a statement, Asda 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, Asda then appealed against this decision on ten 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.
Leigh Day, which represented the staff, said the judgement was a “major step forward in the fair pay battle”.
Leigh Day represents more 30,000 shop floor staff from the big four supermarkets – Asda, Sainsbury’s, Tesco and Morrisons – in similar cases.
The solicitor said the claims against the big four supermarkets, if they lose their cases and are ordered to pay all eligible staff, could total more than £8bn.
The GMB union, which represents some Asda workers, welcomed what it described as a “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.”