The research, launched to mark Work your Proper Hours Day today, found teachers and lawyers are the most likely to do unpaid overtime with around one in five employees putting in 17 hours of free work every week.
Today marks the day when the average person who does unpaid overtime would start to get paid if they did all their unpaid overtime at the start of the year. But the TUC analysis shows those doing ‘extreme' unpaid overtime of more than 10 hours per week wouldn't start getting paid until 26 April.
Last year over five million people across the UK clocked up an average of 7 hours 12 minutes of unpaid overtime a week worth £27.4 billion - or £5,402 each.
One in four (25.3%) public-sector workers worked unpaid overtime in 2009, worth nearly £9 billion a year, compared with one in six staff in the private sector (18.3%). Public- sector workers are also more likely to do ‘extreme' unpaid overtime.
Single women are more likely to do unpaid overtime than single men, married or cohabiting couples or lone parents - with over one in four women putting in an average of 7 hours 42 minutes free work a week.
But another piece of research from Sage UK revealed employees working longer hours unpaid receive no thanks for their trouble.
The Sage UK Omnibus surveyed 2,000 small and medium-sized enterprises and found that over half (52%) of the businesses questioned said their employees worked overtime, with 40% of those businesses not rewarding their staff for working over their contracted hours.
The growth of ‘extreme' unpaid overtime over the past year comes as the number of underemployed people also increases. Official figures show that 2.8 million people say they want more hours in their existing job or full-time work instead of their current part-time job.
TUC general secretary Brendan Barber said: "One of the features of the recession has been people moving to shorter hours or taking part-time work in order to avoid the dole queue. This has also led to a fall in the number of people putting in extra hours at work.
"Our analysis also 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.
"Staff are understandably doing all they can to help their company recover from the recession - and bosses should thank them for going that extra mile. But working time still needs to be properly managed. A long hours culture is bad for workers' health and family life - whether the hours are paid or not."
Brendan Flattery, MD of Sage's Small Business Division, added: "Many employees feel the need to work long hours, but it is important that people continue to strike the right work-life balance.
"By working smarter and using the productivity enhancing software and tools available, firms can really empower their employees. By embracing the appropriate tools, businesses will be able to maintain a more positive and more productive workforce that is not being over stretched and overworked."
