Teachers work more overtime than any other public sector workers, equating to #163;7 billion of free labour for schools a year, according to an analysis of official statistics carried out by the Trades Union Congress (TUC).
The study reveals that 55.4 per cent of primary teachers routinely work overtime, clocking up on average 12.6 hours each week. In secondary, 61.2 per cent regularly do an average of 10 hours' overtime a week.
The TUC has calculated that, in 2012, teachers worked a total of 325 million unpaid hours, worth #163;7 billion in total. This far outstrips the overtime of workers in the health, legal, media and business sectors.
Kevin Courtney, deputy general secretary of the NUT, said that teachers are "manifestly overworked" and their jobs are becoming "increasingly stressful".
"Michael Gove must understand that unless this onslaught against teachers' pay, pensions and working conditions stops then strike action is inevitable," he said.
TUC general secretary Frances O'Grady said the figures highlight "the huge amounts of unpaid overtime teachers do every year - and how much the economy, schools and students benefit from this additional commitment".
"While most staff don't mind doing a few extra hours, working time needs to be properly managed and employers mustn't pressurise their staff into doing more for less," she added. "A long-hours culture is bad for workers' health and family life."
In September, Ofsted chief inspector Sir Michael Wilshaw suggested that teachers who are "out the gate at 3 o'clock" should be paid less than their more industrious colleagues.
Education secretary Michael Gove has repeatedly voiced his support for the school day being extended. In January 2012, he said: "We are all in favour of longer school days, and potentially shorter summer holidays."
When asked about the likely impact on teachers, he replied: "If you love your job then there is, I think, absolutely nothing to complain about in making sure you have more of a chance to do it well."
But the new figures suggest that, for many teachers, putting in the equivalent of an extra working day after hours each week is par for the course.
The statistics came as "no surprise" to Chris Keates, general secretary of the NASUWT teaching union. "Excessive workload and working hours are consistently raised by teachers as their key concerns," she said.