A group of 17 workers are fighting Tesco's decision to reduce their pay on weekends, bank holidays and evenings.
Law firm Leigh Day has contacted Tesco to ask how many employees are affected, but have estimated that 50,000 staff face a pay cut. Most of the staff who will have their wages reduced are over 40.
Paula Lee, a Leigh Day lawyer who is acting for the group, 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 new was leaked to the national press in January 2016."
She said the pay cuts were a "bitter pill" for Tesco's long-serving employees and that the firm believes it is discriminatory.
There seems to be a growing trend amongst retailers to cut staff wages and use those savings across other areas of the business.
It is the longer serving staff in these retail organisations who usually suffer. Understandably our clients feel their loyalty is being taken advantage of; that the employer knows they are unlikely to leave their jobs and in turn that makes them feel vulnerable to further 'no choice' pay cuts.
A spokesperson for Tesco said: “Earlier this year we announced a pay increase of up to 3.1 per cent for colleagues working in our stores across the UK, in addition to a five per cent 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.”
Marks and Spencer also recently cut pay for staff working on Sundays and bank holidays, following the introduction of the national living wage. Changes to pay and pensions at the retailer have landed it with a one-off cost of £100m.