Tesco is facing legal action from staff who say they lost out on pay for working anti-social hours.
The complaint is from 17 workers who are “extremely unhappy” at seeing their pay rates change for weekends, bank holidays and night shifts.
Leigh Day, the law firm acting for the workers, estimated thousands of long-term Tesco staff, mainly in their 40s, could be affected.
Tesco said workers would receive a “transition payment” for the changes.
The employees have started the process towards making a claim in an employment tribunal, according to Leigh Day.
Pay changes announced by Tesco in February included an hourly wage rise, but also cuts to the rates paid to some Sunday and bank holiday staff.
Under the changes, staff would receive time and a half for Sunday and bank holiday shifts from July, whereas previously some had received double time.
A Tesco spokeswoman said: “Earlier this year we announced a pay increase of up to 3.1% for colleagues working in our stores across the UK, in addition to a 5% turnaround bonus.
“As part of the pay negotiations we also agreed to simplify premium payments to ensure a fair and consistent approach for all colleagues.
“The minority of colleagues who were negatively impacted by this change were supported with an agreed lump sum transition payment.”
‘Hurt, bewildered, frightened’
But Paula Lee from Leigh Day said its clients, who have worked for Tesco for at least 16 years, felt “their loyalty was being taken advantage of”.
She said: “Our clients are extremely unhappy that they have had their wages reduced in this way.
“These long-serving employees are especially angry that they only found out about the decision when news was leaked to the national press in January 2016.
“They feel hurt, bewildered and frightened that this could happen again.”
The law firm claims a night premium was also scrapped.
Last Friday, workers at another supermarket, Asda, won a step forward in their battle for equal pay in a case where Leigh Day also acted for staff.
Manchester Employment Tribunal ruled that the women, who mainly work at check-outs or stacking shelves, can compare themselves to higher-paid men who work at warehouses.
The ruling means that test cases will now be brought.
However, Asda said it “continued to strongly dispute the claims”. It said the tribunal’s decision was a “technical” issue and did not “determine the eventual outcome of the case”.