Teachers will need to apply for extra criminal record checks every time they sign on with a new supply agency, under plans being considered by the Government.
The Home Office is consulting on ending the "portability" of Criminal Records Bureau checks, which cost teachers at least pound;34.
Currently teachers apply for one check at their first agency which they show to others. Teachers who get permanent jobs should not be affected as they usually get checked every three years or when they start at a new school.
A consultation letter from the Home Office says it is concerned applicants could commit offences after their first check. But the Recruitment and Employment Confederation, which represents supply agencies, says ending portability would be costly and time-consuming for supply teachers, who often sign up with several agencies.
Tom Hadley, REC spokesman, said agencies agreed more could be done to tighten security but suspected the proposal was simply a money-making exercise. "Repeating checks on teachers isn't necessarily going to make the system safer," he said. "The opposite could be the case because of the risk of overloading the system with paperwork."
Although the CRB is doing checks faster than in its first year, when delays left schools without staff, agencies report some still take as long as six months. The Home Office said no decisions on checks had been made.
A new system for registering school staff is expected this year in response to the Soham murders by caretaker Ian Huntley, who obtained a clean check despite a string of sex allegations.