Supermarket giant Asda is offering staff a higher wage in return for agreeing to work bank holidays and move to unpaid breaks.
About 135,000 staff will be on a basic rate of £8.50 an hour, £1 higher than the new National Living Wage figure from April.
The company said the new contract benefited customers by having greater levels of flexibility, adding that 95% of workers would be better off if they accepted.
Staff could be asked to work in different parts of their store, or work different days or hours depending on when customers shop in their store most.
The new contract, which is voluntary, includes an agreement to work bank holidays if required or to take them as annual leave. It also makes all breaks unpaid and includes changes affecting night shifts.
Hayley Tatum, of Asda, said: "Our current employment contracts have evolved over decades.
"They mean we have different colleagues on different terms and they don't give our colleagues the level of flexibility our customers need to meet their changing needs.
"Our customers shop in different ways at different times and they expect us to deliver them the same great level of service whenever they visit us.
"This new contract will also mean that colleagues can gain a broader level of experience across their store, which will in turn give them better opportunities to progress and develop their career in retail.
"This new contract offer is the right thing for us to do for our customers and our colleagues and we hope that as many colleagues as possible choose to accept it."
GMB general secretary Tim Roache commented: "These new flexible contracts will help to ensure job security, ensure those accepting them are on the same terms and - best of all - ensure that people will earn more money as a result.
"The new contract offer involves quite a few changes but, as it's voluntary, this allows colleagues to choose whatever suits their circumstances best."