The number of Tesco shop workers taking part in a legal challenge to secure equal pay, which could lead to the supermarket paying out £4bn, has swelled to 1,000.

The law firm Leigh Day has filed a further 900 claims at the employment tribunal on behalf of shop assistants, on top of the 100 who started legal action in February, as first reported by the Guardian.

The workers, three-quarters of whom are women, say they earn up to £3 an hour less than mostly male Tesco warehouse workers in similar roles. Up to 200,000 shop-floor staff could be affected by the claim, which could cost Tesco up to £20,000 per worker in back-pay over at least six years.

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Tesco warehouse staff earn from about £8.50 an hour, up to more than £11 an hour, while store staff earn about £8 an hour in basic pay, according to the claim. The disparity could mean a full-time distribution worker earning over £5,000 a year more.

Paula Lee, one of the lawyers handling the Tesco claims at Leigh Day, 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 The concept of ‘women’s work’ is an outdated approach to employment from the middle of the last century which needs to be corrected.”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 spokesperson 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.”

The Tesco case follows similar actions against Asda and Sainsbury’s, which are working their way through the employment tribunal process.

Nearly 20,000 people are involved in the Asda case, where the latest ruling backed the shop workers’ right to compare their jobs to employees – mainly men – working in distribution centres. Asda is due to appeal against that ruling at the court of appeal in October. About 1,000 workers are involved in the Sainsbury’s action.