Scotland could soon be hit by a teacher shortage crisis due to an ageing workforce, it has been claimed.
A teaching union has warned of a crisis in schools
The Scottish Secondary Teachers' Association said age-related illness among older teachers could give rise to a surge in demand for supply teachers.
The association said urgent measures must be taken in the short-term.
However, a Scottish Executive spokesman said the situation was constantly being monitored and he insisted that numbers were increasing.
General secretary of the SSTA, David Eaglesham, said: "Steps must be put in place now to address this problem before it becomes a crisis.
"It is well documented that the age profile of teachers is hugely skewed to the top end and that the 'bulge' is moving close to the point when significant drops in teachers numbers will occur.
"Whilst there has been some coverage of the need to replace those retiring teachers and the problems associated with this, much less has been said about
the short-term impact as this generation moves towards retirement."
He said increasing levels of age-related illness among older teachers would change the balance of supply needs in schools over the coming year.
Mr Eaglesham said: "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."
The association urged its members not to cover classes of absent colleagues
beyond the limits of their local or national agreements.
Mr Eaglesham said: "All of our members will act professionally as always and ensure the best interests of young people are always paramount."
"This will not include, however, interminable additional cover of classes to the detriment of provision for 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."
The executive spokesman said: "We closely monitor the number of teachers joining and leaving the profession to ensure we have the right number.
"We also know how old our teachers are, which helps us plan for the future."
He recognised there were difficulties in the management of Scotland's
11,000-strong supply teaching base, but said the executive was working to
develop best practice guidance for local authorities.