Shopworkers' union Usdaw is seeking recognition at Marks and Spencer after it claimed it had been inundated with calls from angry staff wanting to join the union after discovering they faced cuts in pay.
M&S has announced it would cut its overtime rate from double to time-and-a-half from next year.
The union's deputy general secretary John Hannett said: "Staff at M&S are unhappy. They have been asking us about their rights."
Usdaw had just begun targeting M&S, which does not recognise unions in the UK, in a recruitment drive.
But M&S pointed out it had just increased its basic rate of pay for customer assistants by 3% and was introducing a new package that would extend holiday allowances and give staff shares for every year of service.
The company's 65,000 employees have also just shared in bonus payouts totalling more than £40m.

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