Law firm Leigh Day is reported to have launched legal action on behalf of nearly 100 female shop workers amid claims they earn as much as £3 an hour less than male warehouse staff despite the value of the work being comparable.This  could end up costing the retailer as much as £4bn.

If the legal challenge demanding parity is successful, thousands of shopfloor staff could receive back pay of up to £20,000 each.

Paula Lee, a Leigh Day lawyer who is representing the Tesco women, said: “We believe an inherent bias has allowed store workers to be underpaid over many years. There might be lifting and carrying in the distribution centre but there is also lifting and carrying in shops as well as dealing with customers asking questions and handling money.”

Leigh Day said the underpayment could apply to 200,000 of Tesco’s workers, the majority of them women. It has lodged initial claims with the conciliation service, ACAS – the first stage in what is likely to be a protracted legal process through the employment tribunal system which could last several years.

Reports said that even if a small proportion of the women are successful, the cost to Tesco would be significant.

The retailer stated that it has yet to receive details of any claim, with a spokesperson saying: “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.”