Teachers are working an average of more than 11 hours above their contractual 35-hour week, a survey has found.
The survey by the Educational Institute of Scotland (EIS) asked more than 3,500 primary and secondary teachers to record their working hours over a two-week period.
It found that the average teacher works 46.5 hours per week - an extra 33 per cent on top of their contractual commitment.
The teaching union described the situation as "unsustainable" and called on the Scottish Government and local authorities to act.
In the primary sector, the average teacher spent 9.3 hours on preparation and correction, where the contractual position for a full-time teacher sets aside 7.5 hours.
In addition, planning took, on average, 3.4 hours per week for primary teachers, assessment took 1.3 hours and reporting consumed 2.3 hours of a teachers' time.
In secondary schools, teachers spent 8.5 hours on preparation & correction, 2.5 hours planning, 2.7 hours on assessment and 1.5 hours on curricular development.
EIS general secretary Larry Flanagan said: "It is clear that the current situation is unsustainable and that urgent steps must be taken by the Scottish Government, local authorities and national education bodies to reduce the crippling workload burden that is being placed on Scotland's teachers.
"The workload picture is similar for teachers in all sectors and at all grades, and for teachers on full-time contracts or part-time contracts.
"No matter what the individual contractual commitment, the evidence gathered during the fortnight focus demonstrates that high levels of additional working are the norm for Scotland's teachers."
Scottish Labour said the country would not close the attainment gap with overworked, exhausted teachers.
Education spokesman Iain Gray said: "These findings are very worrying. As a former physics teacher I know full well how hard our teacher's work, but to work the equivalent of more than an extra day per week is simply unacceptable.
"Every teacher accepts the occasional late night as part of the job but this is above and beyond that, this should concern us all because Scotland will not be able to close the attainment gap with exhausted and overworked teachers.
"The SNP have squeezed funding on education in Scotland harder than the Tories have in England, they abandoned promises to cut class room sizes and there are 4,000 less teachers in our classrooms since the SNP came to power.
"The SNP's record on education is littered with broken promises and failures. We have seen in recent weeks they are failing college students, university students, pupils and parents - now it is clear they are failing our teachers too."
But a Scottish Government spokesman said it was working hard to tackle the issue.
"The report by the Working Group on Tackling Bureaucracy (set up by the Government) identified specific areas where changes need to be made and how we can make sure teachers have the freedom they need to carry on delivering our world-class curriculum," he said.
"We are committed to making sure councils have the right number and highest quality of teachers in our schools which is why we have offered councils £51 million, including an additional £10 million over and above last year's settlements to support teacher numbers."
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