Employment Judge Ryan has ruled that women in Asda stores will be able to compare themselves to higher-paid men working in the supermarket's distribution centres.
The ruling means 7,000 Asda workers will be able to proceed with claims against Asda. It will be the UK's largest private sector claim for equal pay, and could involve the business paying out more than £100m worth of pay going back to 2002.
Leigh Day said it was representing women who "feel they have been paid less than others within the organisation despite carrying out roles of equal value".
Lauren Lougheed, a lawyer in the employment team at Leigh Day, said:
This is a dramatic victory for the workers we represent. Asda tried to argue that because the shops and distribution centres were in different locations, with different pay arrangements, that Asda could pay the men what they like.
However, the employment tribunal found that Asda, the employer of both men and women, could have made sure that there was equal pay between men and women if they wanted to, but chose not to.
She said the judgment will have an impact on a similar case that the law firm is bringing for 400 Sainsbury's employees in a similar situation.
An Asda spokesperson said: “This decision by the employment tribunal does not determine the eventual outcome of the case. It relates to a technical preliminary issue of whether jobs in different parts of the business can be compared.
"The tribunal has yet to consider whether the jobs are of equal value in terms of their demands and if some jobs are, only then will the tribunal move on to consider the reasons for the differentials, including the existence of different market rates in different industry sectors. We continue to strongly dispute the claims being made against us. We believe that the demands of the jobs are very different and are considering our options for appeal.
"At Asda hourly-paid colleagues doing the same job in the same location are paid the same."