John Edmonds, president of the TUC and general secretary of the GMB general union, warned ministers that the movement was capable of mounting demonstrations in London more than twice the size of the recent 100,000-strong countryside march and was prepared to do so to make its point.
Addressing the TUC's women's conference in Scarborough, he said unions failed to appreciate the extent to which they would need their lobbying skills after Labour's election victory, and he urged ministers to honour the "substance and the spirit" of a manifesto pledge to introduce union recognition laws.
The CBI believes it has the ear of ministers on the issue and that considerable hurdles will be placed in the way of unions winning negotiating rights at reluctant companies.
The proposed legislation is to be a key element of the Fairness at Work White Paper to be published in May.
In an interview with Radio 4's Today programme Mr Edmonds said: "This is very important for us. It's a clear manifesto commitment that everybody understood. I think the Labour government will understand that if it didn't deliver that there would be some very difficult consequences within the party." Unions still command half the votes at policy-making conferences.Reuse content