This weekend rules will come into force that will protect temporary workers from exploitation and give them the same holiday and pay as full-time members of staff.
But many staffing agencies fear that employers are unaware of the impending changes, that will become law on 1 October.
The TUC general secretary, Brendan Barber, said: 'These new rights for agency workers are an important step forward in helping the UK's hundreds of thousands of agency temps get a fairer deal at work.
'For too long, agency workers have faced discrimination at work. They are frequently paid less, are required to work excessive hours with no overtime pay, and are entitled to less holiday than directly employed workers doing exactly the same job.'
But Virginia Mellers, who co-founded public sector agency Axon Resourcing, said that the changes would 'increase costs of employing agency workers, and with the additional burden of compliance it is very likely to reduce the number of opportunities for agency workers.'
She added: 'With the UK economy fragile, the benefits of being able to mobilise a flexible workforce should be something that companies should be able to make the most of through the recovery – [the regulations] arguably will do little to aid this. This legislation, which has been introduced to provide increased protection to agency workers, may therefore have a polar opposite effect, in that it may reduce the volume and duration of these opportunities.'