Asda bosses have begun a Supreme Court equal pay fight with supermarket workers.

More than 30,000 Asda store workers, most of whom are women, have brought equal pay claims after complaining that staff working in distribution depots unfairly get more money.

Five Supreme Court justices on Monday began to consider whether Asda supermarket staff were entitled to compare themselves to distribution staff for equal pay purposes.

Nearly four years ago, an Employment Tribunal judge decided that supermarket staff were entitled to compare themselves.

That decision was upheld by Court of Appeal judges in 2019.

Asda bosses say the roles are not comparable and want Supreme Court justices to overturn the ruling by Court of Appeal judges.

Lord Pannick QC, who is leading Asda’s legal team, told justices that the issue was whether Asda’s hourly-paid retail store workers could bring equal pay claims because they were “in the same employment” as Asda’s hourly-paid distribution workers.

“Asda submits that the answer is no,” he said, in a written case outline.

“Under domestic legislation, a claimant in an equal pay case may not compare her work and her pay with that of another employee who is employed, even by the same employer, in a different establishment unless the terms of those doing the claimant’s work are common irrespective of the establishment, or type of establishment, at which they work, and the terms of those doing the comparator’s work are common irrespective of the establishment, or type of establishment, at which they work.”

He said terms of working conditions depended on the type of establishment at which people worked and added: “The different types of establishment operate in different geographical locations, in different industries and with different pay-setting processes.”

Lawyers say if store staff win they could be entitled to several years’ back pay.

They say the fight will have implications across the industry and might lead to supermarkets paying out around £8 billion.

Justices are considering rival arguments at a virtual Supreme Court hearing due to end on Tuesday.

Law firm Leigh Day has been instructed by bosses at the GMB union and is representing Asda supermarket workers.

Lawyer Lauren Lougheed, an employment law specialist at Leigh Day, said she was hopeful that supermarket staff would win the Supreme Court fight and “prove once and for all that the roles are comparable”.

Leigh Day lawyers say the supermarket workers’ fight will not end, even if Supreme Court justices rule in their favour.

The employees will still have to show that supermarket and distribution roles are of equal value, and that there is no reason other than sex discrimination for pay differences.

A Leigh Day spokeswoman said if the Asda supermarket staff won, there would be implications for all major supermarkets.

She said lawyers believed that, if the Asda supermarket staff won – and 500,000 eligible staff across the industry made successful claims – then supermarket bosses could owe a total of £8 billion compensation.