Tesco is facing a demand for almost £4bn in back pay from female shopfloor employees, a law firm is claiming.

The grocer could be forced to grant female workers a payout of up to £20,000 – the UK’s biggest ever equal pay claim – if the legal challenge is successful, The Guardian reported. 

Law firm Leigh Day has launched the suit on behalf of 100 Tesco employees. It said it has begun submitting claims through Acas, the conciliation service, as the first step before heading to an employment tribunal.

According to the claim, female shopfloor workers earn up to £3 an hour less than Tesco’s male warehouse workers.

It states that Tesco warehouse staff currently earn anywhere between £8.50 to £11 per hour, while shopfloor workers earn just £8 in basic pay.

If successful, the suit could result in a payout for up to 200,000 workers.

Solicitor Paula Lee said it was a problem that had been “hiding in plain sight” for years.

“We believe an inherent bias has allowed store workers to be underpaid over many years,” she said.

“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.”

The case against Tesco follows similar suits launched against Asda and Sainsbury’s.

In the Asda case, the authority sided with shopfloor workers, backing their right to compare their wages to those of warehouse staff.