Agency workers are set to gain additional rights in pay and benefits under new rules that come into force on Saturday.
Workers will gain similar rights to permanent staff once they have completed 12 weeks of service doing comparable work.
Business groups have suggested the changes could cost firms up to £2bn a year.
There are an estimated 1.4 million agency workers in the UK.
Stefan Martin, an employment lawyer with law firm Allen & Overy, said: "It won't give them equal rights in terms of protection from dismissal."
"What it is going to give them is equal rights in relation to pay and other basic employment rights.
"It's going to be extra basic pay, [and] extra shift allowances potentially, where those workers are not paid at the same level as the equivalent permanent employee," he told the BBC.
RightsVarious legal protections are already in place for agency workers, as they are with permanent full-time and part-time staff. They include the minimum wage and basic holiday rights.
