They claim schools are increasingly turning to temps as regular staff are trained in the literacy, and numeracy strategies and information technology.
David Hart, general secretary of the National Association of Head Teachers, said: "There's been the most fantastic growth in the demand on schools to release teachers for all manner of training on the back of government initiatives. This has increased the demand for supply teachers. I should think the agencies are doing a roaring trade."
The growth is despite serious concerns about the quality of lessons taught by temporary staff. According to an inspection report - due to be published shortly - one quarter of their secondary-school lessons are unsatisfactory.
Meanwhile, teacher supply agencies have seen a huge increase in business over the past 12 months.
The TimePlan agency said this week that its turnover had risen from pound;14 million to pound;18m in the year up to April and that its work had expanded from 117,000 to 150,000 teacher days a year.
Another agency, Select, has more than doubled its business since 1997 and now supplies the equivalent of 7,000 teachers a week while Recruit Education Services says business has grown by 150 per cent in 12 months.
London-based LHR Education has reported a 42 per cent rise in business over the past year.
Most of the agencies believe that schools are demanding more supply staff. However, their expansion may also point to the steady decline of their main competitor - the council-run pool of supply teachers.
John Dunford, general secretary of the Secondary Heads Association, said that changes in employment law - giving employees security of contract after only one year of work - will make temporary staff even more popular.
He said: "Headteachers are going to be increasingly cautious about taking people on full-time unless they can be certain about their quality."
Tish Seabourne, managing director of TimePlan, said: "Since local management of schools was introduced, schools have been able to vote with their wallets, choosing to recruit good supply teachers from whoever supplies the best. LEAs have been losing out."