The number of staff working more than 10 hours a week in unpaid overtime shot up by 14,000 last year.
Research conducted by the Trades Union Congress (TUC) found nearly 900,000 workers undertook 'extreme' overtime last year.
More than five million people across the UK clocked up an average of seven hours and 12 minutes' unpaid overtime a week worth £27.4bn, or £5,402 each.
The figures come as the TUC today marks 'Work Your Proper Hours Day' to highlight that if the average person who does unpaid overtime did all of their extra work at the start of the year, today would be their first paid day of work.
But those who do more than 10 hours a week in unpaid overtime would not start getting paid until 26 April.
Teachers and lawyers were found to be the most likely to do more than 10 hours of unpaid overtime, with about one in five employees working an extra 17 hours a week for free.
In the public sector, 25% of staff worked unpaid overtime in 2009, worth nearly £9bn a year, compared to 18.3% in the private sector.
Public sector workers were also more likely to do 'extreme' unpaid overtime, according to the TUC analysis.
Single women were more likely to do unpaid overtime than single men, married or cohabiting couples or lone parents, with more than one in four women putting in an average of seven hours and 42 minutes' free work a week.
Brendan Barber, general secretary of the TUC, said: "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."
He added: "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."