Mandate says the company has informed them that it wants to move almost 1,000 long-serving staff to new contracts
A union has warned Tesco it will ballot its more than 14,000-strong workforce for industrial action if it forges ahead with plans to cut some workers' pay.
Mandate has written to the supermarket chain warning that it could face strikes if it refuses to reach agreement.
It said the company has informed them that it wants to move almost 1,000 long-serving staff to new contracts by April 19.
Assistant General Secretary of Mandate Gerry Light says the company has to decide how it intends to proceed.
Trade union SIPTU has said it 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.
It would mean cuts of more than €2 an hour for some staff.