Almost a quarter of teachers in England are working 12-hour days, according to a leaked government report.
Some 22% of teachers said they were working 60 hours a week or more.
The research, commissioned by the Department for Education (DfE), was carried out in Spring 2022 and has not yet been made public.
Education Secretary Gillian Keegan has said a new taskforce will be created to help reduce teachers' workload by an average five hours a week.
Teaching unions are currently consulting members in England on a new pay offer, which includes the promise to reduce their workload.
The leaked document - marked confidential and titled Working Lives of Teachers and Leaders - was produced for the DfE to examine issues around teacher supply, recruitment and retention.
