The number of Tesco store staff lodging an equal pay challenge against the grocer has swelled to 1,000.

A further 900 claims have been filed by law firm Leigh Day at an employment tribunal on behalf of shopfloor workers. The claims come in addition to the 100 shop assistants who began legal action against the supermarket giant back in February.

If the store staff are successful, Tesco could be faced with an eye-watering £4bn bill in back pay.

As previously reported by Retail Week, the shopfloor workers, three-quarters of whom are women, claim they earned as much as £3 an hour less than their colleagues employed in Tesco’s warehouses.

Fulfilment centre staff earn an hourly rate ranging from around £8.50 per hour to more than £11 per hour, compared to around £8 an hour for store staff, Leigh Day has claimed.

It says the disparity could mean full-time distribution workers earn more than £5,000 a year more than their shopfloor colleagues.

Up to 20,000 store staff could be in a position to make similar claims, which could cost Tesco as much as £20,000 per worker in back pay over at least six years.

Leigh Day lawyer Paula Lee said: “We’ve had an incredible response to the announcement of this legal action. Many proud members of staff have realised that this claim is not anti-Tesco, but it is to ensure that the work done in stores and distribution centres is recognised as being of equal value – not the same work, but work of equal value.

“And that they should be paid the same as their colleagues in distribution.

“The concept of ‘women’s work’ is an outdated approach to employment from the middle of the last century which needs to be corrected.”

A Tesco spokeswoman said: “We cannot comment on an ongoing legal matter, but we will be defending this claim about equal pay.

“Tesco has always been a place for people to get on in their careers, 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.”

Leigh Day is also representing Asda and Sainsbury’s shopfloor staff on similar claims, which are currently working their way through the employment tribunal process.

Almost 20,000 staff are involved in the Asda case, while around 1,000 have made claims against Sainsbury’s.