UK workers will put in more than £23 billion of unpaid overtime this year, according to a new report.
Britons lose "billions" in overtime
The TUC said around five million people work an average of seven hours and 24 minutes without pay every week - worth £4,500 a year.
Using official statistics, it said that 1.5 million managers were working unpaid overtime.
Professional staff were averaging nine hours 36 minutes a week extra, worth £9,000 a year, the union said.
The research showed that 150,000 craft workers were averaging an extra six hours a week, worth £3,000 and 70,000 plant and machine operatives were doing an additional five hours 36 minutes of unpaid work which should give them over £2,000 a year.
The TUC has launched an online calculator to show people the hours they should work and what they lose in unpaid overtime.
TUC general secretary Brendan Barber said: "Britons work the longest hours in Europe, and these figures show that much of it is unpaid overtime.
"We're not saying we should turn into a nation of clock-watchers, or that no-one should put in extra work when there's an emergency or a rush of orders, but many people are clearly putting in the equivalent of an extra day every week.
"Is it any wonder that top jobs are still dominated by men, when managers have to do an extra day's unpaid work each week?
"When employers quote dubious figures about the costs of what they call red tape, and everyone else calls basic rights at work, do they remember their staff put in billions of pounds worth of unpaid extra work each year?
"Given that workers in much of the rest of Europe work fewer hours, yet produce and earn more, are there not hard questions to ask about the quality of UK managers?"
But John Cridland, deputy director general of the CBI said: "The TUC is painting a skewed picture of employee attitudes to working time to get more pay for its members.
"In reality the unions not only dislike unpaid overtime, they also want to limit paid overtime by removing the right to opt out of the working time directive.
"Whether paid or not, the message seems to be 'we know best' on the number of hours people should work.
"The fact is most people do extra hours because they want to."
The TUC published the figures as part of its campaign for the Government to end its opt-out from a European directive aimed at limiting hours to 48 per week.