More than 700 temps, freelancers and consultants were working in IPA member shops last year - up 11 per cent on 2002, new census figures show.
Meanwhile, the number of full-time agency employees has fallen for the third successive year to 14,036.
As a result, the average number of people employed in 220 IPA member agencies now stands at 64, one fewer than in 2002 and five fewer than the 2000 figure.
Mary Budd, the IPA's employment affairs adviser, welcomed the trend towards more flexible working hours in the industry. "This should allow us to respond better to the challenges of the market and reduce the cost and pain of redundancies," she said.
The survey also suggests that agency employees are not only fewer in number but getting younger. Forty-eight per cent of agency staff are aged 30 or under, while just 5 per cent are over 50. The average age of agency staff is 33.