Workers would lose paid meal breaks and have to work bank holidays but get a pay rise under new terms.
GMB, the union for ASDA workers, has called on the supermarket giant to negotiate over contract changes affecting almost 60,000 workers.
Under the new terms, workers would no longer be paid for their meal breaks and would have to work bank holidays – but would see their pay bumped up to £9 an hour.
The contacts already exist on a voluntary basis – but ASDA is now considering imposing them on employees.
GMB urges ASDA to stop this approach, and negotiation the changes with staff properly.
Gary Carter, GMB National Officer, said:
“Absolutely nothing has been agreed with GMB and we will fight any imposition of these contracts on our members.
"Since ASDA introduced its flexible contract two years ago, nearly 60% of employees have opted not to go on the new contract.
“These contract changes will affect nearly 60,000 members of staff they cannot just be imposed from the top.
“We expect ASDA to negotiate. If they want to roll out the new contract they must listen to employees and sit down with GMB to discuss beneficial improvements to terms and conditions which have the support of the workforce.
“We know ASDA employees will be anxious and unsettled by the announcement, which comes shortly after CMA provisional findings."