The Government's decision last week to include casual and agency staff under the Part-Time Work law could dramatically push up labour costs.
The long-awaited regulations, which give part-time workers comparable terms and conditions to full-timers, will apply to the wider definition of "worker" rather than just "employees".
The CBI said it is disappointed the Government has broadened the scope of the law, which comes into force on 1 July, but said the effect will be tempered because part-timers' benefits will only have to equal those of full-timers with the same status. This means part-time agency staff and freelancers will gain the same benefits as full-time agency staff and freelancers, not full-time permanent staff.
"In a way we have won the more important victory," said Susan Anderson, head of employee relations at the CBI.
"If we had the 'worker' definition, without the same-status clause, then any part-time, casual worker could waltz in claiming the same rights as a full-time member of staff, but that will not happen."
But there are concerns that this will still mean bigger bills for organisations using a large number of agency staff.
Sally Storey, director of HR at Bournewood NHS Trust, said it will push up the cost of hiring agency nurses and care workers.
"Part-time staff employed by the NHS already have the same rights as full-timers but most NHS trusts spend significant amounts on agency employees.
"This will have a similar impact to the Working Time regulations when we had to pick up the bill for annual leave. Any extra cost will be passed on to us."
The use of temporary staff in the NHS in Scotland alone runs to £25m a year.
The TUC has welcomed the decision, but said it would like the law to go further.
Julia Edwards, employment partner at law firm Edge Ellison, said employers need to make sure their part-timers are not being treated less favourably than full-timers in similar jobs.
But she added that there is still nothing in the law to compel employers to offer or promote part-time work - the central tenet of the European directive.
Employers will have eight weeks, as promised by Steven Byers, to implement the law.
What the law says