The Technical Engineering and Electrical Union has said it is advising members to vote No to the Lisbon Treaty.
The national executive of the TEEU, which is the country's largest craft union, has said it is advising its 45,000 members to vote No on 12 June.
It says it favours a social Europe, but it says recent key judgements by the European Court of Justice show that the pendulum has swung against workers' rights and in favour of big business.
