Acas’s report, Three sides to every story: the impact of the Agency Worker Regulations, found agency workers were unaware of rights around holiday pay, notice period and the 12-week qualifying period (the point when an agency worker who is doing identical work to a regular worker is entitled to the same basic terms and conditions).
Analysing calls to its agency workers helpline, Acas found one in five calls were related to contract issues.
The body also found agency workers are afraid to assert their statutory employment rights due to the perceived power imbalance.
Acas chair Brendan Barber said: “Agency workers can feel a sense of insecurity and fear around their contracts, similar to those raised by calls to our helpline on zero-hours contracts.
"We found examples of agency workers who were afraid of questioning their employment rights or completely unaware that they were entitled to some basic rights.
“Some workers also complained that they were asked to take on tasks that they were not qualified to handle and employers wanting to ignore the law so that they could sack people immediately.”
Barber added that the employment relationship with agency workers has a “triangular nature” that has to “balance the needs of workers, hirers and agencies”.