Almost all Asda’s 120,000 workers who are paid by the hour have signed new contracts with the supermarket chain ahead of a deadline it extended to midnight on Saturday.

The retailer, owned by US giant Walmart, has been at the centre of a lengthy and high-profile row over the imposition of the new contracts, which it says help it remain competitive in a “challenging market”.

The hashtag boycottAsda has been trending on Twitter after one employee, Duncan Carson, announced his bid to take legal action against the supermarket, which is being warned that its actions might trigger a consumer backlash in the crucial run-up to Christmas.

Under the new deal, basic pay increases from £8.21 to £9 per hour, and will increase again to £9.18 in April. Asda says workers will receive an enhanced package including an annual bonus.

But the GMB union, which represents the staff, said the new contract removes paid breaks and allows Asda to change workers’ roles, hours, shifts, start and finish times as it pleases.

The union had claimed last month that as many as 12,000 workers were refusing to sign. But Asda said fewer than 250 had not signed – and this number included employees who had retired.

Asda customers overwhelmingly want to be served by people who are respected and treated well in their jobs Gary Carter, GMB official

The supermarket set a deadline for workers to sign the contract by midnight last Saturday. It then extended the window for signing by a week to give staff a chance to think again. The company said that anybody working after last Saturday night has done so on the new terms.

Asda said it was the last of the big retailers to employ such a contract, which it said represented an investment in pay from Asda of more than £80m.

An Asda spokesman said. “We have always been clear that we understand people have responsibilities outside of work and we will always help them to balance these with their work life; we will not be asking them to constantly move the time they work, their days or departments.”

The row could hit the chain in the pocket. A Survation poll of more than 1,000 Asda shoppers carried out for the GMB found 37% – which would equate to 7 million consumers – said they would be less likely to shop at the supermarket after hearing the terms of the new contract.

“Asda customers overwhelmingly want to be served by people who are respected and treated well in their jobs – no one wants to feel bad about doing their shopping, especially at Christmas,” said Gary Carter, GMB national officer for Asda.

“GMB don’t take any joy from this, because our members’ jobs depend on people shopping at Asda but people want to feel good about where they shop and clearly could vote with their feet.”