New figures show the number of supply teaching hours in schools fell by 15 per cent between 2010/11 and 2012/13, with a further drop of 16 per cent in the last school year.
The cut in hours is a result of a well-documented shortage of supply teachers caused by a cut in their wages, which has since been partially reversed.
The Educational Institute of Scotland (EIS) teaching union said the figures highlighted the importance of addressing the issue, which is currently being looked at by a joint group of teachers' leaders, councils and the Scottish Government.
An EIS spokeswoman said: "The payment of supply teachers will be looked at and there will also be a consideration of the interaction between supply and teacher workforce planning. The EIS will pursue and push for solutions."
Ken Cunningham, general secretary of School Leaders Scotland, which represents headteachers, said schools were "heavily dependent" on supply staff to cope with the development work associated with the new Curriculum for Excellence.
A Scottish Government spokesman said: "Staffing levels in schools have been broadly safeguarded over the past three years ... We recognise that schools and local authorities have faced challenges securing supply cover and that's why the most recent pay agreement made changes to the pay rates for supply teachers."