Britain has the highest number of temps in Europe
Unregulated employment agencies are exploiting the growing number of temporary workers, according to the TUC.
General secretary elect Brendan Barber has said cowboy agencies will continue to exploit and under-pay the vulnerable unless workers have proper rights.
And, in a report published next week, the organisation says the main problem is that there is no requirement for agencies in the UK to be licensed and up to one out of every three have been in business for less than a year.
It is calling for full employment protection for all agency staff - as soon as they start work.
This already happens in other European countries.
Next month an EU directive giving all agency workers equal pay and basic employment rights could be agreed.
But the government wants a qualifying period of between six and nine months.
Employment relations minister Alan Johnson said agency workers already have some legal protection.
But he added the directive would be supported if jobs were not put at risk.
Temps make up 800,000 of Britain's 24 million strong workforce, the highest number in Europe.
Agencies typically charge between 10% and 25% commission for supplying temps and a one-off fee of between £1,500 and £4,000 if they are employed by the company.
Secretarial and clerical workers made up only about 13.5% of temporary and contract workers in 2000 to 2001, compared with 29% three years earlier.
Nearly 9% of temps are managerial and professional workers and about 70% of temps are female, though that percentage is gradually falling.