The union "broadly welcomed" the EU Directive on Agency Workers, but said it was flawed because equal rights on pay, overtime and holidays would only kick in after a "qualification period" of six weeks.

Most agency workers do not work for employers or agencies for that long and would therefore never qualify, argued the union, which said the CBI and British government wanted to block changes.

"There is a growing army of agency workers in print and packaging, and some reputable job agencies," said Tony Burke, the GPMUs deputy general secretary. "But we know of agency workers only just getting the national minimum wage. Many dont get holidays, overtime or shift allowances. Some face safety risks."

Union spokesman John Abbott added: "Employers dont like anything that restricts their right to hire people one day and send them home the next. We want to stop the ability to undercut permanent employees by taking on agency staff at low rates."

The GPMU said 18% of UK temporary workers were agency staff.

The dates for when the new EU directive comes into force are still to be decided.