Morrisons is the latest major supermarket group to be challenged over equal pay for its shop floor workers in a claim which could cost the retailer an estimated £1bn if successful.

The law firm Leigh Day said it was seeking compensation for shop workers – predominantly women – who believe they are paid less than the mostly male workers in Morrisons’ warehouses.

Leigh Day said it would lodge a claim with the employment tribunal service next week on behalf of eight Morrisons shop workers, but added that around 80,000 employees could be eligible to claim back pay totalling more than a billion pounds.

The latest move means Leigh Day is now bringing equal pay claims against all of the UK’s ‘big four’ supermarkets. It has already begun legal action on behalf of 30,000 staff working in Asda, Sainsbury’s and Tesco stores.

The law firm said warehouse workers were paid between £1 and £4 an hour more than their colleagues in shops at other major supermarket chains.

“We believe that Morrisons, as with the other major supermarkets, has underpaid those working in its stores for a number of years,” said Emma Satyamurti, a partner in law firm Leigh Day’s employment team.

“Our clients believe that those working on the shop floor should be paid the same as those in the distribution centres, and a failure to commit to this is not only unfair but unlawful.”

The law firm argues that although work carried out in stores and warehouses is different, the work should be recognised as being of equal value.

Claims have already been lodged with the conciliation service Acas, Leigh Day said, and it has written to the retailer’s chief executive, David Potts, requesting pay and gender information for workers.

A spokesperson for Morrisons said: “We are not aware of any court proceedings issued by a third party. We have received a letter asking us a number of questions about our pay policies. Our aim is to pay our colleagues fairly and equally for the job that they do, irrespective of their gender.”

The latest move follows similar actions against Asda, Sainsbury’s and Tesco, which are working their way through the employment tribunal process.

About 20,000 people are involved in the Asda case, where the latest ruling backed the shop workers’ right to compare their jobs to employees – mainly men – working in distribution centres. Asda is due to challenge that ruling at the court of appeal in October.