Around 1,000 Tesco employees have voted to back strike action over plans by the company to reduce their pay and conditions.
There was an 85% turnout - and 99% of those voted to back industrial action.
Mandate Trade Union assistant general secretary Gerry Light said: "We've now served notice on the company that our members intend to strike in the event the company proceeds with their plans to cut wages or alter the contracts of employment without agreement."
Earlier, the supermarket multinational said it was deferring its plan to reduce pay and conditions for around 1,000 of its 14,000 employees until Monday 16 May.
It also said it had formally opened a voluntary redundancy over the scheme for staff.
Mr Light added: "It's a pity it took until the declaration of our ballot before the company accepted the invitation to attend the WRC, especially when you consider we wrote to the company more than a month ago.
"It seems the company was waiting for the result of the ballot before they agreed to engage, but better late than never."
Tesco has now accepted an invitation to attend the Workplace Relations Commission for a conciliation conference.
Mandate trade union said talks involving unions and management at Tesco will take place at the Workplace Relations Commission next Thursday.
The dispute centres on around 1,000 workers, 6% of the Tesco workforce, who hold pre-1996 contracts of employment involving higher rates of pay and better working hours than those applicable to staff recruited after 1996.
The company argues that the pre-1996 contracts are not sufficiently flexible to meet the needs for longer opening hours and changed trading patterns.
The pre-1996 workers are employed in 89 of the company's 149 stores.
However, Mandate and SIPTU said that workers would stand to lose out due to the lower pay provisions and more flexible working hours in the post-1996 contract.
The "migration" to the lower contracts was set to go ahead this Monday, but Tesco has now deferred that until 16 May.
It has also formally opened a voluntary redundancy scheme offering five weeks pay per year of service uncapped to staff who might prefer to leave.
For staff who choose to remain, moving to the less generous "modern" contract, Tesco would offer compensation totalling two and a half times the annual loss.
Separately, the Labour Court had recommended a 2% pay award, which was paid to all staff except those on the pre-1996 contract.
Tesco has now confirmed that the 2% will be added to the redundancy package of anyone who chooses to leave.
The company has also pledged that additional working hours that might be available after the voluntary redundancy scheme will be offered first to existing part-time staff.
Pre-1996 employees have a contractual entitlement to a 5% bonus.
Newer recruits have an entitlement to a share bonus which was not paid in 2015 due to a deterioration in Tesco's trading performance.
However, the company has now paid a 1.5% share bonus to the post-1996 employees.
In a statement this afternoon Tesco said it was "disappointed" with the result of the Mandate ballot for pre-1996 employees.
"We have proposed a generous compensation offer including a voluntary redundancy scheme at five weeks per year of service uncapped and compensation of 2.5 times annual loss of earnings for colleagues moving to our main contract.
"We are proposing to move these colleagues to our main contract, which already covers the vast majority of our workforce, as the pre-1996 contract means we have too many colleagues working during the early quieter times of the week and not enough during the busiest," the statement said.
Talks at the Workplace Relations Commission were adjourned last month without agreement.
