The State's largest craft union, the Technical Engineering and Electrical Union, has urged its 45,000 members to vote "No" in next month's Lisbon Treaty referendum.
TEEU general secretary designate Eamon Devoy said recent judgments by the European Court of Justice demonstrated that the pendulum had swung against workers' rights and in favour of big business.
In these circumstances, it would be "foolish" to give more power to EU institutions.
The decision by the TEEU, which represents middle-income workers in manufacturing, construction, energy, engineering and electrical contracting, will come as a serious blow for the "Yes" side in the treaty debate, which is increasingly worried about the breadth of sectional interests showing hostility to the treaty.
The TEEU is the first major union to take a public stance on the referendum; a number of other unions have said they would like to see concessions from the Government on agency workers in the national pay talks before pledging their support.
The national executive of the Irish Congress of Trade Unions (Ictu) will decide on its stance when it meets on May 18th.
The pro-treaty group, the Irish Alliance for Europe, last night described the TEEU's decision as premature because, it claimed, the issues the union wanted addressed would be dealt with in the treaty.
Its chairman, former Labour party leader Ruairí Quinn, said it was "very much" in the interests of ordinary workers that the treaty be passed. "By voting 'Yes' we will be giving unprecedented protection to Irish workers by enshrining the Charter of Fundamental Rights into EU law," he said.
He predicted other unions would support the treaty.
Explaining the decision of his union's national executive, Mr Devoy said that while recent European court judgments accepted workers' right to organise in unions, they negated this by saying industrial action could not be taken where it conflicted with the provision of goods and services, regardless of the social consequences.
He cited the Laval case, where the court found against Swedish workers who were preventing lower-wage Latvian workers from accessing a building site, and the Viking case, involving a Finnish company which crewed its boats with cheaper Estonian workers.
A third case, known as Ruffert, struck a particular chord with TEEU members, he said. Here, the court found that a Polish subcontractor operating in Germany was entitled to pay workers less than half the agreed minimum wage for the construction sector, because the right to provide unrestricted services took priority over collective wage agreements.
"Twice in recent times we have found Polish workers at Moneypoint being grossly exploited by German contractors and paid as little as €5 an hour. In another instance we discovered Serbian electricians being paid as little as $3.81 an hour. We were only able to ensure proper rates were paid to these workers after strong pressure, including the prospect of industrial action, was exerted on the companies concerned."
The Ruffert judgment raised the spectre of similar abuses of vulnerable migrant workers in future, he said, and would make it "all but impossible" for Irish workers and companies to compete for tenders. Mr Devoy said that until EU states were prepared to recognise the right of workers to take industrial action in defence of living standards", the TEEU would not support "reforms that only strengthen big business".
