Marks and Spencer has been accused of planning "disgraceful" cuts in pay which could cost workers £150 a month.
The Transport and General Workers Union said it will take up hundreds of complaints from workers over reductions in Sunday and Bank Holiday payments.
The union complained that the cuts were going ahead even though the store chain had recently announced profits of £721 million.
"The company is now back on the up, so to turn their loyal workforce and cut back their income is nothing short of a disgrace," said TGWU National Organiser Brian Revell.
A spokeswoman for the company confirmed that during the next financial year payments for working Sundays and Bank Holidays will be reduced from double time to time-and-a-half as part of a number of changes to make the company more competitive.
A number of new financial benefits will be introduced, including a 3% rise on basic pay and extra holidays.
Marks and Spencer, which employs 60,000 workers in the UK, said staff will also be given free shares.
PA