An equal pay case is being taken against supermarket giant Tesco which lawyers estimate could lead to a bill of £4 billion.
Law firm Leigh Day will take the first stage of the claim to the conciliation service Acas this week on behalf of 100 women, claiming they are paid less than men for work of equal value.
The lawyers said the case could eventually involve 200,000 women, making it the largest equal pay challenge in the UK.
Tesco said it works hard to ensure that all staff are paid "fairly and equally."
But lawyers argue that employees working in the male-dominated distribution centres are paid considerably more than the largely female-staffed Tesco stores, and may earn £11 an hour while the most common grade for store staff sees them receive around £8 per hour.
The disparity could see a full-time distribution worker on the same hours earning over £100 a week, or £5,000 a year, more than store staff, Leigh Day said.
The law firm said it had been approached by more than 1,000 employees and former employees of the supermarket.
Paula Lee, from Leigh Day, who is representing the Tesco women, said: "We believe an inherent bias has allowed store workers to be underpaid for many years.
"In terms of equal worth to the company, there really should be no argument that workers in stores, compared to those working in distribution centres, contribute at least equal value to the vast profits made by Tesco, which last year had group sales of £49.9 billion."
A Tesco spokesman said: "We are unable to comment on a claim that we have not received.
"Tesco has always been a place for people to get on in their career, regardless of their gender, background or education, and we work hard to make sure all our colleagues are paid fairly and equally for the jobs they do."