The GMB union has urged Asda to negotiate over contract changes that would remove pay during meal breaks and require employees to work bank holidays.
Under the new terms, which would affect nearly 60,000 workers, employees would see their pay increased to £9 an hour.
The contacts already exist on a voluntary basis, but Asda is now considering imposing them on employees.
Gary Carter, GMB national officer, said: "Since Asda introduced its flexible contract two years ago, nearly 60 per cent 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.
He added: “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."