The research by the Association of Teachers and Lecturers (ATL) involved 730 of its independent school members and found that 23 per cent of them are working more than 21 hours extra per week.
Furthermore, a third are working between 11 and 20 hours extra per week, while another third are working up to 10 hours extra.
Eight in 10 of the teachers working longer hours say they are doing so because their workload demands it, while two in 10 say it is their school that is demanding the extra time.
One teacher from the South East said: “I have regularly worked four out of five evenings in the week and at least one and a half days on the weekend.”
Another from the East of England said: “So many more initiatives are generating paperwork, box-ticking, etc, all taking time away from the key tasks of planning, delivering lessons and giving feedback to students.”
One issue raised by the respondents was the expectation that they should be “on call”, with a fifth reporting being expected to respond immediately to parents who contact them.
Also revealed was the pressure on more than a quarter of the teachers to teach outside of their specialist subjects, with a majority of those saying they had not received adequate support to be able to do so.
The ATL’s general secretary, Dr Mary Bousted, said: “Independent school staff, along with other education professionals, face excessive workloads and are working longer hours’ year-on-year. It is especially worrying that so many of those working in independent schools are expected to work extra hours, and that some schools even demand them to.
"Too many schools are not taking work/life balance seriously enough. Independent schools should agree expectations with parents and staff about when staff will respond to messages. Independent schools need to encourage a better work/life balance as a heavy, unmanageable and unrealistic workload is leading to increasing numbers of experienced and valuable staff leaving the profession."