Sainsbury’s could face legal action from the union Unite after the supermarket said it was pressing ahead with a plan to cut paid breaks, annual bonuses and premium pay for Sundays, leaving thousands of workers out of pocket.

After weeks of consultation with staff and pressure from unions and MPs, the supermarket on Thursday pledged extra pay for online delivery drivers, those on night shifts and those in outer London boroughs, adding £10m to its pay settlement.

Sainsbury’s said it was investing £110m in a pay rise for 93% of its 130,000 shop workers, taking pay from £8 to £9.20 an hour, or £9.80 in London, to give an average pay rise of 9.3%.

However, the removal of paid breaks, the annual bonus and Sunday and bank holiday pay means 7% of Sainsbury’s staff, or about 9,000 people, will suffer an average pay cut of £400 a year. The supermarket said it would make up the difference in their earnings for 18 months and pledged to review its policy in March 2020.

Unite said it had anticipated that a consultation about the new pay arrangements would run until 4 June and feared staff who did not sign up to new contracts enshrining the pay changes could be made redundant in September when they come into effect.

The union said it was seeking legal advice over whether Sainsbury’s had failed to meaningfully consult workers. It also claimed Sainsbury’s was attempting to harmonise its contracts with those of Asda workers before a planned merger, which was announced in April.

Joe Clarke, the union’s national officer for food and drink, said: “Bosses have cut short the consultation extension over the plans, which will leave thousands out of pocket while still holding a gun to workers’ heads with the threat of sign up to the new contract or be out of a job.”

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A spokesperson from Sainsbury’s said the company had informed Unite on Wednesday that there were no grounds to continue the consultation and was “disappointed with this reaction from Unite as they have had a seat at the table throughout the consultation process”.

They said there was no legal merit to Unite’s proposed claim.

“This consultation relates to Sainsbury’s stores only and has nothing to do with the proposed merger with Asda,” the spokesperson added.

Usdaw, the shopworkers’ union, said it remained concerned that some workers would lose out but said it was pleased to have persuaded Sainsbury’s to make some concessions after “extensive consultation”.

Sainsbury’s has previously been accused by unions of “robbing Peter to pay Paul” and more than 100 MPs signed a letter to the prime minister this week calling for her to intervene.

The MPs, led by Labour’s Siobhain McDonagh, criticised the retailer for a “whole array of deplorable decisions that will hit hardest their most dedicated, loyal and long-term staff”.

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Sainsbury’s said the new deal was “intended to rectify the unfairness of colleagues working side by side in store, doing exactly the same job but being paid different amounts depending on when they started working for Sainsbury’s.”

Simon Roberts, the retail and operations director of Sainsbury’s, said the supermarket had tried to minimise the negative impact of the changes and denied that the majority of those affected by pay cuts would be longer-serving staff.

“We are trying to do the right thing and to protect our business and grow our business, and protect jobs going forward,” Roberts said.