survey suggest the true picture is far worse for many teachers, revealing how much of their time is spent tackling tasks such as lesson planning, marking and data entry.
Headteachers are even more stretched, the figures show. While 15 per cent of classroom teachers clock up at least 21 hours of overtime a week, this rises to 24 per cent among assistant and deputy headteachers and 31 per cent among school leaders.
More than half of classroom teachers (52 per cent) said they worked at least 12 additional hours each week.
"This survey just reinforces why it's so important that we tackle excessive teacher workload," Ms Morgan told TES. "Our Workload Challenge received tens of thousands of responses from teachers right across the country and we're now working to turn their suggestions into concrete action."
According to the School Teachers' Pay and Conditions Document (STPCD), teachers are required to work for a maximum of 1,265 hours per year. This equates to about 32 hours a week during term time, but the figures suggest that the vast majority of teachers work for substantially longer. Academies and free schools are officially permitted to deviate from the STPCD.
Rob Campbell, principal of Impington Village College in Cambridgeshire, said the proportion of teachers routinely clocking up 21 or more additional hours a week was a cause for significant concern.
"That worries me, if they are spending so much time planning and marking," he added. "We don't just need to jettison individual tasks, it's about introducing cultural change in schools. We need a more radical, reflective cultural solution."
Kevin Courtney, deputy general secretary of the NUT teaching union, said the figures showed that many teachers were working "an extra half a working week" during their evenings and weekends.
"It's deeply disturbing," he added. "We're hearing about more and more teachers giving up on the job because they can't cope with the working hours. We think the extra hours are caused by an accountability system which has gone badly wrong and shows a lack of trust in teachers."
Earlier this month, TES reported that many teachers were being swamped by marking, owing to the increased focus on assessment in Ofsted's latest inspection framework (" `Assessment treadmill' may overwhelm staff", 14 November). Inspectors no longer grade individual lessons, instead judging teaching on factors such as the setting of homework and the quality of marking, assessment and feedback.
Brian Lightman, general secretary of the Association of School and College Leaders, said the "long-hours culture" in schools was "unsustainable".
"A tired and stressed teacher or school leader can't do the quality of job which we would want them to. The danger is that it is making the profession less attractive, and our concern is the impact on recruitment. The survey reflects the vast workload, which has increased dramatically in recent years and has been exacerbated by the rate and pace of reform," he said.
Teachers were often expected to complete large amounts of paperwork in a "perpetual state of readiness for Ofsted", Mr Lightman added.
`I work 30 extra hours a week'
Rob Campbell, principal of Impington Village College in Cambridgeshire, routinely works 30 hours of overtime a week.
"School doesn't start until 9am but I'm in at 7am," he says. "I'm rarely away until 6pm and then I work most of Sunday, so I'm probably working close to 30 extra hours a week.
"That's probably comparable with senior executives and chief executives in the world of business."
But Mr Campbell, a member of the Headteachers' Roundtable group of influential school leaders, is more concerned about classroom teachers working excessive hours. Accordingly, he has developed a system of flexible working as well as a new marking policy to ease the pressure on staff.
"We work hard here to ensure the burden on teaching staff doesn't get too much," he says. "I don't say they have to be in school if they don't have lessons; they can work at home. We use different forms of assessment and try to limit the number of meetings.
"I don't like to keep people here needlessly at the end of the day."