Tesco — the UK’s biggest supermarket — is facing a demand from thousands of mainly female shop workers for back pay that could leave it with a bill of up to £4bn ($5.5bn).

Law firm Leigh Day has launched legal action against the supermarket giant on behalf of nearly 100 shop assistants who claim they are paid up to £3 an hour less than their male warehouse workers, the Guardian reported.

According to the law firm Tesco warehouse staff earn between £8.50 to £11 an hour while shop workers earn about £8 an hour in basic pay.

Lawyers have claimed the value of the work is comparable so the pay should be the same.

It’s thought up to 200,000 staff could be affected by the claim, which could cost the supermarket up to £20,000 per worker in back pay over at least six years.

Why it matters?

Equal pay means staff doing the same job must be paid the same amount — it is a legal requirement. However the interpretation and implementation of this law has been the subject of debate, with many groups claiming women face pay discrimination in the work place.

The BBC is amongst employers being criticised for paying men and women differently.

All UK companies with over 250 employees have to publish data on the gender pay gap by 5 April.

Overall, the UK’s gender pay gap for all employees, full-time and part-time, stood at 18.1 percent in 2016, down from 20.2 percent in 2011, according to data from the Office for National Statistics.

What was said:

Paula Lee, of Leigh Day solicitors, told the BBC it was time for Tesco to tackle the problem of equal pay for work of equal worth.

We believe an inherent bias has allowed store workers to be underpaid over 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 the depots, contribute at least equal value to the vast profits made by Tesco.

The law has been there since 1984 – you can compare with a different job. That’s 34 years to put your house in order; that’s 34 years of having the advantage of paying unequally, 34 years of you making pay decisions and making financial decisions and 34 years hiding what is in open sight.

A Tesco spokesperson 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.

Background:

Two other UK supermarkets — Asda and Sainsbury’s — have been hit with similar claims in recent year which are working their way through the employment tribunal process.

Some 1,000 Sainsbury’s workers are involved in action against their employer, while nearly 20,000 people are involved in the Asda case.

The latest ruling against Asda backed the shop workers’ right to compare their jobs to employees working in distribution centres.

The UK’s public sector has also become embroiled in pay equality claims.

Birmingham City Council is has been found liable for over £1bn pounds in payments after settling an equal pay claim from women employed as cleaners, cooks and carers.