In a warning of risks to come, the report predicts employment will decline in "union sectors" like manufacturing and the utilities and grow in public services and the predominantly non-union business services.

A study by the Workplace Employee Relations Survey, cited in the TUC report, says the failure of trade unions to win membership and recognition in new workplaces is a key reason for union decline.

Outdated image

The TUC report also shows unions suffer from an outdated image, with non-members concerned unions are troublemakers.

And a new poll by the London School of Economics suggests there are 5m working in non-union workplaces who would like union representation and another 3m in unionised workplaces that unions have not yet recruited.

That totals the same as the 8m membership detailed in the latest official figures.

TUC general secretary John Monks said the report was a "challenging document for the movement".

"Few other institutions would have the courage to produce this warts and all account of their strengths and weaknesses."

He said the report provided insight into how existing unions could reach their full potential in membership.

It outlined the kind of initiatives for a "new model of unionism for employees we cannot reach."

The ideas will be presented to next week's TUC conference but the decision on whether to press ahead will be made at the organisation's general council meets in October.

Key strategic win

Mr Monks told BBC News Online he would taking the chance next week to make the case for trade unions.

One key success he will highlight is the acceptance of the European Directive On Information and Consultation that "will introduce works councils into all large and medium firms in Britain".

"I regard this as absolutely crucial strategically to the future of British trade unions.

"I see this as a platform on which to build 'a voice for every workplace' and that is the slogan for Congress this year."