The Scottish Secondary Teachers' Association has repeated its advice to members not to cover for absent colleagues beyond the limits of their local or national agreements - but the Scottish Executive says it is up to local authorities to manage teacher supply.
The union is issuing a revised and updated information leaflet to schools early this term.
The advice has been prompted by SSTA concerns about teacher shortages and supply cover problems due to the increasing age profile of the teaching profession.
David Eaglesham, the SSTA's general secretary, said: "Increasing levels of age-related illness among older teachers will change the balance of supply needs in schools over the coming year. Demand for cover will increase at a time when available supply is already at a low ebb.
"Already many schools find severe difficulty in obtaining replacement staff for short and medium-term absences, with consequent disruption for schools.
Steps must be put in place now to address this problem before it becomes a crisis."
Mr Eaglesham said that members will "act professionally as always", but this will not include interminable additional cover of classes to the detriment of a teacher's existing workload. "No rational person would ask airline pilots or train drivers to continue to do additional hours beyond the contractual limits for fear of the consequences. Teachers should be treated likewise," he said.
A spokeswoman for the Executive said: "We closely monitor the number of teachers joining and leaving the profession to ensure we have the right number of teachers. We also know how old our teachers are, which helps us plan for the future. Far from decreasing, our teacher numbers are actually increasing and we are committed to reaching 53,000 by 2007.
"Supply teachers have a vital role to play in helping local authorities deal with absence. There are around 11,000 supply teachers in Scotland - the issue is not the number of teachers but the management of them. We are aware that there are difficulties with the current system and are working to develop best practice guidance for local authorities."
The Executive points out that teachers take less time off sick than other council employees (3.9 per cent against 5.3 per cent for administrative and professional grades and 6.4 per cent for crafts and manual).
An allowance for supply teachers is built into the national teacher workforce planning exercise and this element was increased last year from 5 per cent to 6 per cent in primary and 7 per cent in secondary. "Local authorities are responsible for managing absence among their workforce," the spokeswoman said.