The militancy that governs the words and actions of trade union leaders threatens to make the organisations "irrelevant", a former TUC economics chief warns today.
In a new pamphlet from the Fabian Society, David Coats warns that unions are "stuck in the past, fighting battles in a class war that is of little relevance to most people today".
The decline in union membership is evidence of this - it has fallen from more than 12 million in 1979 to 6.4 million today, and Mr Coats warns this will continue unless unions change the way they operate.
In addition, he claims this decline threatens the link between trade unions and the Labour party, as it becomes increasingly undemocratic to have these smaller organisations having such a large say in the development of party policy.
This shift is already in evidence - last month trade and industry secretary Alan Johnson, himself a former union leader, suggested that the union vote at the annual party conference be cut from 50 per cent to just 15 per cent. It was 90 per cent in 1994.
"The Labour party will certainly benefit from a strong link to trade unions that are representative of the workforce and valued by employers, whose legitimacy is not in question," said Mr Coats, now associate director of the Work Foundation.
"This requires union renewal. Just as the Labour party had to go through a painful exercise of political reinvention in response to social and economic change, so unions must embark on a similar journey or run the risk of continued marginalisation and eventual irrelevance."
Today's pamphlet, Raising Lazarus: The future of organised labour, says that 62 per cent of workers support a collective voice in the workplace, but many potential union members are put of by the rhetoric of strife and struggle.
It recommends government and unions make more of an effort to work together, and abandon the pattern "of unions making an unrealistic demand and the government responding with a resounding 'no'".
In the past few months, the leaders of all the public sector unions threatened a mass strike of three million workers over pensions, while this week the Public and Commercial Services (PCS) union balloted members on industrial action over civil service job cuts.
But Mr Coats recommends that unions spend less time threatening action on pay and equality and more time on working with employers to address the problems.
"In their mutual distrust, unions and government are missing the opportunity to build a progressive consensus in the workplace, to address questions of low pay and equal pay, income inequality, working time and flexibility, training and skills, anti-discrimination and the role of worker-voice institutions," he said.
However, PCS general secretary Mark Serwotka rejected the suggestion that unions were declining or stuck in the past, saying that his own organisation and the RMT were both renowned for their campaigning, and were Britain's two fastest growing unions.
"As we've campaigned and stood up against rogue employers to fight for equal pay against job losses, to keep work in the public sector, our unions have grown phenomenally," he told Today.