Staff at Marks and Spencer, once seen as one of the most generous employers, are furious to discover that their pay packets are likely to be getting smaller.
M&S has angered its shop assistants
The retailer has decided to axe its policy of paying staff double time on Sundays, Bank Holidays and for evening work.
The change, which will be introduced at the start of the next financial year, means workers will instead receive time and a half for working unsociable hours.
The Transport and General Workers' Union called the cuts a "disgrace" and estimated the cost to each worker who depends on such payments could be around £150 a month.
The union, which is pressing for official recognition, said it would take up hundreds of individual grievances on behalf of the workers.
"We encourage M&S workers in any region to contact their local T&G office to join so that together we can defend the wages and conditions of M&S workers," Brian Revell, T&G National Organiser said.
A spokeswoman from M&S defended the company's decision, saying it brought the firm more into line with the norm of other retailers.
And she said that the pay for shop assistants in M&S stores was still in the top three of all British retailers.
A number of new benefits are also to be introduced, including share options and more holiday, as well as the annual pay rise of 3%.
The shake-up in pay and benefits comes as M&S is starting to turn its business around and is reporting healthier profits.