The TUC report shows 5.25 million people worked overtime in 2008 without pay - the highest number since records began in 1992.
On average, if employees were paid for the extra work they do, they would receive £5,129 a year - this means if they did all their unpaid work at the start of the year they would not be paid until 27 February.
The TUC encourages employees to work their proper hours at least once a year (on Friday, 27 February) and for employers to regularly thank them for the extra work they put in.
General secretary of the TUC Brendan Barber said: "The number doing unpaid overtime has increased for the second year in a row. This is disappointing.
"The recession will now be making people scared of losing their job in the year ahead and joining the ever-growing dole queue. Inevitably people will be putting in extra hours if they think it can help protect against redundancy or keep their employer in business.
"The recession should instead provide a spur to make workplaces more productive and for managers to get staff to work together, not compete for who can stay the latest."
But Bridget Biggar, managing director of Management Intelligence Consulting, thinks the TUC has missed the point on overtime.
She said: "Unpaid overtime is a sign the UK's workforce is banding together to do their bit to help haul us out of economic nosedive - and in many but admittedly not all cases [staff] are working harder because they enjoy it.
"If somebody is enjoying their role, is stimulated and energised by the stretch of the challenge that uses their personal skills and strengths, they will work longer hours without even realising - and certainly not begrudge it."
Unpaid overtime saves UK employers 26.9 billion
More than five million people are working unpaid overtime in the UK, saving employers 26.9 billion in salaries, according to the TUC.