Temp workers should have the same rights as other members of staff, the Trades Union Congress (TUC) has said.
Currently, there are ongoing discussions about an EU Temporary Agency Directive, which the TUC is asking the government to urge the EU to sign up too.
At the moment, temp workers do not have the right to the same pay and working conditions as permanent employees.
TUC General Secretary Brendan Barber said: "Agency workers regularly earn less than directly employed staff, are not allowed to benefit from an employers contributions to a pension scheme, are given less holiday, little if any access to training and tend to get no contractual sick pay.
"Some people, including women and older workers, are always going to prefer to work a series of temporary contracts."
In time, a whole "underclass of temporary workers" could be created by employers who are increasingly replacing staff with contracts with temp workers, Mr Barber added.
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