Teachers in Scotland are working an additional 11 hours a week on top of their contracted hours, according to a survey by a union.
The EIS asked 3,500 primary and secondary teachers across the country to record their working hours over a two-week period.
It found that the average teacher works 46.5 hours per week. They are contracted to work 35 hours every week.
The union said teachers were being burdened with a "crippling" workload.
It described the situation as "unsustainable" and called on the Scottish government and local authorities to act.
The Scottish government said it was working with teachers' representatives, local authorities and other partners to address concerns around workload.
A spokesman said: "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.
"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 £51m including an additional £10m over and above last year's settlements to support teacher numbers".
The EIS Fortnight Focus on Workload survey found the average primary school 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 and correction, 2.5 hours planning, 2.7 hours on assessment and 1.5 hours on curricular development.
EIS general secretary Larry Flanagan said: "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's education spokesman Iain Gray said: "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."