Teachers are more likely to do overtime than any other professional group, according to figures released today.
More than half of teachers work unpaid overtime, with in excess of one in five working an average of 19 extra hours a week, research published by the Trades Union Congress (TUC) shows.
The figures reveal that a higher proportion of teachers work overtime than lawyers and health service managers.
The analysis was published to mark today's Work Your Proper Hours Day. Last year more than 5 million people across the UK clocked up an average of seven hours, 12 minutes of unpaid overtime a week, worth #163;27.4 billion - or #163;5,402 each - the TUC said.
The number of people working "extreme" overtime of more than 10 hours a week increased by 14,000 to nearly 900,000.
Kevin Courtney, deputy general secretary of teaching union the NUT, said: "These figures confirm what we already know: that the pressure for long hours keeps going up.
"It's not just hours. There is a clear link with the high stress levels among teachers and it really is time for the Government to take its duty of care seriously.
"Teachers have always done long hours, but they used to be long hours preparing exciting lessons for kids; now for many they are long hours on paperwork for accountability purposes."
The research shows that workforce remodelling - which introduced guaranteed planning, preparation and assessment time - had failed to cut long hours, he added.
The findings follow the results of a government-sponsored survey last year, which showed that almost all teachers are working more than 50 hours a week.
Secondary classroom teachers are working an average of 50.4 hours a week compared with 51.3 in 2000, it showed. Primary teachers are working 51.2 hours a week, only slightly down from 52.8 in 2000.
The TUC figures claim that one in four public sector workers did unpaid overtime in 2009, worth nearly #163;9 billion a year, compared with one in six staff in the private sector.
Brendan Barber, TUC general secretary, said: "Our analysis disproves the tired stereotype that public sector workers enjoy a feather-bedded working life.
"In fact, they are giving away billions of pounds worth of work for free - and more per person than those in the private sector."
A spokesman for the Department for Children, Schools and Families said that progress was being made on reducing teachers' hours.
But he added: "We are aware that this isn't the case across the board and more needs to be done to ensure that employers are implementing the workforce agreements we have agreed with the unions."
The introduction of "rarely cover" rules and record numbers of support staff were helping to address the problem, he said.