Temporary agency workers could have their careers hit because of a lack of employment rights, the TUC warned.
Astudy by the organisation showed that half of agency workers were in a job for six months and one in four for more than a year.
But the TUC said temps were denied the same level of training as other employees, leaving them "trapped" in low paid, insecure jobs.
The "gulf" in training risked creating an underclass of temporary workers without adequate skills, said the report. The TUC urged the Government to encourage the EU to agree a Temporary Agency Directive improving rights to agency workers.