Mr McMillan added: "Supply staff used to be deployed for sickness. The need has now burgeoned so that staff development is a major claim. The result is that we cannot get the cover because it seems to have become accepted that staff development must take place during pupil hours."
Officials are now taking a series of steps to improve the management of staffing. These include better planning where staff development requires "significant cover".
The city and Moray House Institute are also mounting a concerted drive to attract returners, with an in-service course for 25 secondary and 30 primary teachers beginning next Saturday.
But the council has also told heads to help themselves by avoiding activities requiring cover during the peak period for staff illness from December to February.