Nearly 10 years' of improvement to teachers' workloads is being eroded as they spend more time buried in paperwork, official figures published this week show.
The research, undertaken by the School Teachers' Review Body, shows the first across-the-board increase in teachers' working hours since 2000.
That was the year the Government sought independent advice on reducing teachers' spiralling workloads, culminating in an agreed list of administrative tasks that should be delegated to support staff. It has now emerged that this year's rise has seen classroom teachers working their longest hours since 2004 - and not much of that time is spent in the classroom.
Faye Craster, 22, a science teacher from north London, said she was working up to 65 hours a week.
"It's been the hardest year of my life," she said. "If I was earning Pounds 45,000 a year, I wouldn't mind so much. I'm not getting half that."
In July, at her north London comprehensive, Ms Craster bade farewell to 25 of her 70 teaching colleagues. Many cited the workload and stress as reasons for leaving. She said time that she would like to spend planning is spent on "banned" tasks such as data input, bulk photocopying and cleaning. "After a lesson, I spend half an hour washing and putting away 100 test tubes, then I have 10 sinks to clean," she said. "My blind broke the other day, so I fixed it myself.
"And the school is desperate to improve its league-table ranking. The student intake isn't changing, so the teachers just have to work themselves silly."
Ms Craster expects to serve only one more year, then take her science and communication skills into academia or industry.
"Maybe I'm being selfish, but I'm not staying for the love of the children or the school. Something has to go, because I can't do a 65-hour week every week," she said.
Many teachers began 2008 with a sense of dread, describing it as "meltdown year" because of the many policy and curriculum changes. The new school year sees the introduction of the new diplomas, revisions to A-levels and the new secondary curriculum.
The Department for Children, Schools and Families admitted a workload problem could be emerging, but blamed it on schools breaking rules on work time. Ministers have told officials to investigate.
Beverley Hughes, the children's minister, said they had worked hard to free heads so they could run their schools and teachers to teach.
"By law, no teacher is now required to do administration tasks," she said. "There is a strict statutory limit on the amount of cover for colleagues, and every teacher must have half-a-day dedicated preparation time a week. Ultimately, it is down to employers themselves to support their staff properly."
A diary survey by the review body shows primary teachers work 52.2 hours a week, and secondary teachers work 49.9 hours.
Most spend fewer than 20 of those hours teaching. The rest, they say, is spent on tasks that should not be part of their jobs.
Two years ago, a government report said headteachers were wasting their time unblocking toilets. Now, teachers say they are spending hours every week on cleaning and photocopying, rather than spending time with children, teaching, preparing lessons and assessment.
The NASUWT union warned that most teachers were working well beyond the 48-hour maximum set by the European Union.
"We do have concerns that nearly a quarter of primary headteachers are doing things not related to their role, in particular undertaking caretaking responsibilities," said Chris Keates, the general secretary.
Christine Blower, her counterpart at the NUT, said: "Below-inflation pay increases and excessive working hours is not a recipe to make teaching attractive."
A two-part series on teacher workload starts in The TES on September 5.