SIPTU have said they will not accept proposals from Tesco to cut the wages of long serving staff at the company.
Tesco intends to change the employment conditions of all staff employed since before 1996.
This will hit about 1,000 workers and will mean cuts of more than €2 an hour for some staff.
Mandate trade union, which also represents Tesco workers, has said staff will be at least €6,500 worse off a year after the changes.
Tesco has been accused of making the changes to increase profits.
The company has said they are making the changes to increase the flexibility of the workforce.