London: Supermarket group Tesco is facing a potential bill of up to £4 billion ($5.6 billion) to bring the wages of its female employees into line with men, according to the law firm pursuing Britain’s largest equal pay claim.

Tesco is Britain’s biggest retailer and its largest private sector employer with more than 310,000 staff.

Law firm Leigh Day said on Wednesday the mainly male staff in the company’s distribution centres were paid considerably more than its largely female store workers. The law firm said it was also working on claims at supermarket rivals Sainsbury’s and Asda, the British arm of Walmart.

Unequal pay for men and women is currently a hot topic in Britain’s boardrooms and corridors of power. The resignation last month of Carrie Gracie as China editor for the BBC led to an investigation into pay differences at the public broadcaster.

British Business secretary Greg Clark told Sky News he was “surprised" by the scale of the claim against Tesco.

A Tesco spokesman said the firm had not yet received a claim.

“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," he said.

Tesco shares were down 0.9% at 1010 GMT.

Leigh Day said Tesco distribution centre staff may earn in excess of £11 pounds ($15.4) an hour, while the most common grade for store staff saw them receive around £8 per hour.

This disparity could see a full time distribution worker on the same hours earning over £100 week—or £5,000 a year—more than female store staff.

The law firm said more than 200,000 Tesco employees may be underpaid and estimated shortfalls could reach £20,000, meaning the potential bill for Tesco could be as high as £4 billion.

Leigh Day said it had already started submitting claims on behalf of its clients through conciliation service ACAS, the first stage in the Employment Tribunal process.

It said it had been approached by over 1,000 employees and former employees of Tesco.

“In terms of equal worth to the company there really should be no argument that workers in stores, compared to those working in distribution centres, contribute at least equal value to the vast profits made by Tesco," said Leigh Day employment lawyer Paula Lee.

The firm said it was also representing over 20,000 shop-floor workers in equal pay claims against rival supermarket groups Sainsbury’s and Asda. Reuters