Asda is offering staff a higher wage in exchange for a new contract which will introduce unpaid breaks and a requirement to work over Bank Holidays.
The supermarket group will pay its workers £8.50 an hour, above the £7.50 National Living Wage rate due to come into force in April.
The "flexible" deal means Asda's 135,000 staff can work around the store, on different days and hours.
Asda says signing is voluntary and it is not a zero hours contract.
However, it means that employees must work on bank holidays if the store needs them to, or, if they want to take the time off, it must come out of their 28 days of annual leave.
Additionally, all breaks will be unpaid and Asda will alter its night shift window. Currently, workers are paid an extra £2.04 for unsociable hours that run between 10.00pm and 6.00am.
Under the new agreement, unsociable hours will be cut to between midnight and 5.00am but the premium rate will rise to £2.54 an hour.
Asda, which is owned by US retail giant Walmart, claims that 95% of its staff will be better off under the new deal, which will be introduced in October.
It said it was "maintaining it commitment not to use zero hours contracts and colleagues will be guaranteed minimum hours".
Asda added: "Whilst the new contract will require colleagues to be flexible, fair and reasonable notice will be given for any changes to rotas, and consideration will be given to those with care requirements outside of work."
The UK's third biggest supermarket chain, after Tesco and Sainsbury's, has been struggling with declining sales. In the most recent quarter, over the Christmas period, it reported a 2.9% fall in like-for-like sales.
However, the rate of decline in sales has slowed as it reported steeper drops in previous quarters.
The new contracts have been given the seal of approval by the GMB union.
Its general secretary, Tim Roache, said: "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."