The TUC report, based on figures from the autumn 1998 Labour Force Survey, shows that unions are signing up more women, part-time and Asian workers. Unions are also signing up members in traditionally low membership sectors, such as sales.
John Monks, TUC general secretary, said: "This report shows that unions' recruitment and organising efforts are starting to pay off. But unions must not be complacent - in almost half the UK's workplaces there are still no union members."
The study shows that 6.8 million people - 30 per cent of UK employees - are union members. The figure is unchanged on last year.
The greatest increase has been among women members, which have risen by 60,000 since 1997, partly as a result of the increasing number of part- time employees in trade unions.
The TUC said the figures reflect new membership drives among affiliate unions, such as the partnership agreement between Tesco and the shopworkers' union USDAW, which has seen membership among Tesco staff increase by 9,000.
Nonetheless, unionisation remains highest in the public sector, running at 60 per cent compared to only 19 per cent in the private sector.