Tesco employees who say they have lost out on pay for working anti-social hours are considering legal action against the supermarket.
Law firm Leigh Day said it is representing 17 long-term employees who claim they have seen their wages reduced for working at weekends, bank holidays and nights.
Pay changes announced by Tesco in February saw an overall hourly wage rise which was accompanied by cuts to the rates paid to some Sunday and bank holiday staff from July.
Paula Lee from the law firm said the group, who have worked for Tesco for at least 16 years and are mainly in their 40s and above, felt like "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."
In February the supermarket announced a pay increase of up to 3.1%, taking the hourly rate of its shop workers to £7.62, ahead of the Government's introduction of the national living wage in April.
The package, which Tesco said it agreed with union Usdaw, also included a store discount, pension and a 5% turnaround bonus.
The supermarket said all staff would be paid time-and-a-half for Sunday and bank holiday shifts from July.
A small number of staff had received double time for these shifts, Tesco said, adding that 85% of its estimated 250,000 hourly-paid staff affected by the changes would be better off under the new arrangements.
A night premium between 10pm and midnight was also allegedly scrapped, Leigh Day said.
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."
The law firm said conciliation talks were ongoing and if these failed it would issue an indirect discrimination claim on behalf of its clients.
It said it is also representing thousands of employees in equal pay claims against Asda and Sainsbury's.