Illegal deductions mean many agency workers fail to earn the minimum wage, according to a report published by the TUC today.
A growing number of employment agencies are illegally charging inflated transport, accommodation, clothing and meal costs to the temps on their books, it said.
The report comes the week before the Council of Ministers is due to debate the European Temporary Agency Workers Directive.
It said regulations that came into force in 2003 to protect agency workers from exploitation are doing little to prevent abuse, especially of migrant workers.
Dodgy agencies routinely make deductions for cashing pay cheques and for providing clothing and safety equipment, said the report.
This leads to hourly rates for many agency workers falling to well below the £4.85 set by the minimum wage.
TUC general secretary, Brendan Barber, said most workers stayed silent because they were unaware of their rights and terrified of losing jobs and accommodation.
“The current regulations which are designed to protect agency workers from abuse at the hands of rogue agencies are clearly doing nothing of the sort,” he said.
Barber said the government should give its full support to the European Directive so that the legal standing of UK agency workers was improved.