Nearly five million UK staff worked an average extra day a week in unpaid overtime in 2005, according to the TUC.
The union group estimated that if each employee worked all their unpaid overtime at the start of the year, they would not get paid until 24 February.
But its analysis of official labour statistics also found that the percentage of people working unpaid overtime was at its lowest since 1992.
Staff in small workplaces were least likely to work extra time without pay.
The TUC's analysis of the government's latest Labour Force Survey found that 4,759,000 workers, or 19.4% of employees, worked an average of 7 hours 24 minutes in unpaid overtime each week.
Londoners put in the longest hours, with those doing unpaid overtime putting in an extra 8 hours 12 minutes a week.
Extra effort
They were followed by workers in Wales (7 hours 48 minutes) and Northern Ireland (7 hours 36 minutes).
"We need the support of the employee to not put their palm out for every penny for every moment they work"
"We don't want to turn into a nation of clock watchers," said TUC general secretary Brendan Barber.
"Most people enjoy their jobs, and don't mind putting in extra effort when there's a rush or in an emergency, but that easily turns into the long hours culture of extra hours every week."
Alan Addis, managing director of Verdict Aerospace Components, which supplies commercial jet maker Airbus, told the BBC that a bit of unpaid overtime was essential for companies such as his to compete in the global manufacturing market.
"We need the support of the employee to not put their palm out for every penny for every moment they work," he said.
"They look at it as an investment into the long term future for themselves.
"If we can reduce our costs it gives us some really sound longevity."
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