Supreme Court justices are preparing to consider the latest leg of a long-running equal pay dispute between bosses at Leeds-based Asda and 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.
Lawyers say if supermarket 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.
Supreme Court justices are today (Monday 13 July) due to consider whether Asda supermarket staff are 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.
Five justices are listed to consider the case at a virtual Supreme Court hearing over two days.
They are not expected to deliver their ruling until later in the year.
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 ruled 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 their fight, 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.