Britain's largest retailer is facing a legal challenge over a pay gap between its store workers (mostly women) and its warehouse staff (mostly men).

A group of 100 mostly female store workers has accused supermarket chain Tesco (TSCDF) of paying them as much as £100 ($140) a week less than employees in its distribution centers.

Leigh Day, the law firm that represents the store employees, argued that the workers are of "equal worth" to Tesco and should be paid the same.

"There really should be no argument that workers in stores contribute at least equal value to the vast profits made by Tesco," said Paula Lee, the lead lawyer from Leigh Day.

She added that it was time for companies to "see the [gender] inequality which is still deeply entrenched in their organizations."

The claim, which asks for back pay of up to £20,000 ($28,000) per store worker, could put Tesco on the hook for a record £4 billion ($5.6 billion).

Tesco, which said it has not yet received any claims, said that it "works hard" to make sure all of its employees are "paid fairly and equally for the jobs they do."

Related: Investors warn companies: Hire more women!

According to the claims, distribution center employees typically earn over £11 ($15.30) an hour, while store workers get only £8 ($11) per hour. The annual difference is £5,000 ($7,000).

Leigh Day has started submitting claims to an employment tribunal, the first step in what could be a lengthy legal process.

Pedro Martins, a professor of applied economics at Queen Mary University of London, said that occupations dominated by women tend to have lower average wages.

But Martins said that to win the legal argument, the store workers must prove they are contributing the same value as warehouse workers, and that their skills and hiring circumstances are similar.

"The big issue is the comparability of the two types of occupations," he said. "If a firm struggles to recruit and retain workers for a specific occupation, it will have to increase the wages they pay," he added.

Tesco has been collecting data on its gender pay gap since 2002. Its most recent report showed the median pay gap between men and women workers was 8.6%.

Related: Men taking pay cuts won't fix the wage gap

Gender pay gaps are high on the political agenda in the U.K.

A law that came into force last year requires businesses with 250 or more workers to publish information about their pay gap by April.

The "name and shame" tactic is designed to force companies to close the gap. There are signs the approach is already having an impact.

Six male BBC stars took pay cuts after the publicly-funded broadcaster disclosed data on its gender pay gap.

And the CEO of EasyJet (ESYJY) said last month he would lower his salary to £706,000 ($982,000) -- the same earned by his female predecessor.