Mandate has warned that Tesco's plans to remove established terms and conditions for staff moving to a new store at the same location in Cork could have nationwide implications.
On RTÉ's Morning Ireland, Mandate's Lorraine O'Brien said the decision by the retailer to erode terms and conditions could be 'the start of something bigger'.
She said the union believes that workers throughout the retail chain are at risk of something similar happening to them.
Workers at the Tesco store in Douglas in Cork voted unanimously last night for industrial action.
Their row centres on plans to change working terms and conditions for workers moving to new premises on the same site.
The ballot of the 85 Mandate members took place at a city hotel last night with almost 90% voted in favour of action.
The new store will employ up to 160 people. The old store currently employs 92.
A small number of workers, known as the pre-96ers, received letters on Good Friday telling them they had to sign the new terms by last Tuesday or face redundancy at the end of this month.
Pre-96ers are those who were working with Quinnsworth when it was taken over by Tesco in 1996.
Mandate says 4,000 of Tesco 12,000 staff in Cork are pre-96ers.
Profits posting
Meanwhile, the world's third biggest retailer showed its resilience to the economic downturn with underlying annual profits of £3.128bn (€3.55bn) - a record for a British retailer.
Tesco does not give a breakdown of how its Irish operations fared, but said it was helped by favourable movements in sterling exchange rates.
It said its Irish operations delivered a 'solid performance' despite the extremely difficult economic climate while trading conditions have worsened in recent months.
It said that a combination of very tight cost control and more international buying have helped it invest in lowering prices for Irish customers.
It said these investments have become even more necessary given the steep rise in cross-border shopping into Northern Ireland encouraged by the decline in sterling in relation to the euro - which has seen an estimated 4% of the total market move over the border.
However, it added that the benefits of its new distribution centre at Donabate in north Co Dublin have delivered substantial efficiencies and improved stock management.
It added that it has seen a strong performance from its new Irish stores.
The company also said it is to create hundreds of jobs in Northern Ireland this year.
The new jobs will come with the expansion of three outlets at Knocknagoney and Newtownbreda in Belfast and Carrickfergus, Co Antrim, which are being turned into Extra stores.
The supermarket chain now has 47 stores in Northern Ireland
The company employs 440,000 people in about 4,000 stores across 14 countries.
