The ability of trade unions to secure better pay for their members has been called in to question by a new study.

Research carried out for the Joseph Rowntree Foundation has found that unionised manufacturing plants were more likely to close than non-unionised plants and that unions failed on average to negotiate higher pay rises.

With Labour granting unions the statutory right to recognition where a majority of the workforce wish, the findings will come as a disappointment to unions.

The tendency of twenty years ago for union representation to inhibit job growth still remains, while unions seem to have more of an effect on the process of pay bargaining than the eventual outcome, argues the report.

Neil Millward, senior research fellow at National Institute of Economic and Social Research and co-author of the report, said it showed that while the ability of unions to enhance wages is in long-term decline, the negative effect of unions on job losses are generally avoidable when they are given a role in determining employment matters as well as pay.

"On the one hand, it seems that employers have little justification for opposing unionisation on the grounds that it might lead to unacceptable wage costs. On the other, unions will find it harder to appeal for new members on the basis that they generally achieve higher pay," said Millward.

Commenting on the report, TUC general secretary John Monks said: "Unionised workplaces have fewer low paid workers, and unions have successfully narrowed the gender pay gap in workplaces where there are trade union members. Employers who work closely with unions tend to have safer workplaces and better trained workforces, and are much less likely to end up at employment tribunals."

The CBI pointed out that the most unionised workplaces tend to be those in traditional manufacturing sectors which have been under the greatest pressures as a result of the strength of the pound and foreign competition and were therefore most likely to see jobs lost and pay deals squeezed.

"While being in a trade union doesn't mean your firm is about to go to the wall, it isn't necessarily the case that being in one means your job is safer because global forces are having a more significant effect," said a spokesman.

The main findings of the study were that:

- Between 1990 and 1998 manufacturing plants were more likely to close if they were unionised, with the one exception of those with a comprehensive bargaining agenda.

- In the service sector there was no differential closure rate between union and non-union companies.

- In the economy as a whole, non-union workplaces grew on average by 1.4 per cent a year between 1990 and 1998 while unionised workplace shrank on average by 1.8 per cent. After considering other factors such as the size of the company, the difference remained significant.

- In 1998 unions failed to negotiate higher pay rises for their members when all factors were taken in to account, although in the private sector they did win higher pay where they represented a large proportion of the workforce or there was more than one union involved.

- In 1998 pay increases were lower when unions covered most employees, suggesting a long-term decline in the ability of unions to enhance pay.

Researchers at the NIESR and the Policy Studies Institute examined evidence from the 1990s during their research into the impact of unions on the workplace.