The Social Protection Minister Joan Burton has described as "disappointing" the decision by Mandate to call for a No vote in the EU Fiscal Treaty referendum next month.
Ms Burton, Labour's Director of Elections for the campaign, has said the debate is only beginning, and it would have been preferable to go through the pros and cons before arriving at a decision.
She has also urged those against the treaty to come up with an alternative.
General secretary, John Douglas, said he would make no apology for campaigning against the Fiscal Treaty ahead of the referendum on May 31.
“The fiscal treaty, if passed, will not create one job,” Mr Douglas warned.
“On the contrary it will legally lock down Irish economic activity at its current levels, and may even shrink domestic demand further leading to mass unemployment, decades of emigration and sow the seeds for future social conflict.
“We strongly urge all our members not only to vote no to the treaty, but to get involved in their local ’no campaign’.”
Mandate is one of the country’s largest unions and represents over 45,000 workers, mainly in the retail trade.
It revealed its decision at its biennial delegate conference in Wexford as a poll showed 55% of the public do not understand the treaty.
The Behaviour and Attitudes poll for the Sunday Times also revealed the yes side was leading, with 42% planning to vote yes, while 27% would vote no, and 31% said they do not know.
Mr Douglas told more than 300 delegates that austerity policies being pursued by Government as part of the ongoing Troika bailout agreement are choking the life out of the Irish economy, particularly the domestic economy. “This treaty has nothing to do with ’good housekeeping’ or ’managing the household budget’,” he added.
“It is about copper fastening into an internationally legally binding agreement, decades of austerity, social exclusion, mass long term unemployment and emigration – and a continuation of attacks on workers’ rights and the welfare system.
“It is not about what is good for Irish citizens, or the citizens of Europe, it is a treaty of the Right for the Right.”
Ms Burton maintained the national debate on the stability treaty was only getting under way.
“It would have been preferable to have a discussion on the pros and cons of this treaty before taking a position,” said Ms Burton, Minister for Social Protection and director of elections for the Labour Party.
“I believe that saying yes to the stability treaty is vital for a stable currency, for investor confidence, for economic recovery, and for a vote of confidence in Ireland’s future.”
Ms Burton said this will be achieved by making sure there is a responsible approach to budgeting, providing a sound basis for investment and growth.
“It is incumbent on those urging a no vote to set out an alternative,” she added.
Meanwhile Minister of State for European Affairs, Lucinda Creighton, said trade unions have a responsibility to reflect on what a no vote will mean.
“I find it difficult to understand why people who represent workers whose jobs are absolutely contingent on the Government’s recovery plan working could possible advocate a no vote,” she said.
“This is a treaty and a referendum which is of absolute direct consequence, and huge importance to, improving stability in the eurozone, in safeguarding and securing our currency which pays the salary of these workers.
“It’s all about confidence and investment, that’s what all of this is about.”