The Irish Congress of Trade Unions (Ictu) has warned it may not support the new EU "reform treaty" unless it gets assurances that the Government will support the charter on fundamental rights.
Ictu president Peter McLoone said reports that Ireland had sought to reserve its right to follow Britain in opting out of the charter were of serious concern to congress.
The charter document contains a range of citizens' rights such as the right to engage in collective bargaining and strike action.
Under the proposed "reform treaty", agreed at the last EU summit in Brussels, the charter would become legally binding under EU law.
In his address to Ictu's biennial conference in Donegal today, Mr McLoone said: "The charter represents a statement of basic values to inform the development of the EU - basic values like respect for human rights and human dignity, democracy, equality, and workers' rights.
"Any form of opt-out would represent a significant shift in the Taoiseach's position - outlined to this conference in 2005 - and would jar incongruously with everything we have tried to achieve on workplace rights in Towards 2016."
Around 600 delegates are attending the a four-day conference in Bundoran, which is due to be addressed by the Taoiseach tomorrow.
Mr McLoone said that over the last quarter of a century, the EU has underpinned many gains in workers rights. "We now have the most right-leaning, employer-friendly commission and council in the history of the European Union," he told delegates.
