More than five million people worked unpaid overtime in 2008, bringing its total value across the UK to a record £26.9 billion, according to a new analysis of official statistics published by the TUC.
The union body warned a recent trend to shorter hours has been reversed and says the economic downturn could increase the pressure to work for free.
TUC has calculated that 5.24 million people worked unpaid overtime in 2008 - the highest number since records began in 1992. The previous record was five million in 2001. Employees who work unpaid would receive an extra £5,139 a year if they were paid for the additional hours they are putting in, the TUC calculated.
The average amount of unpaid overtime is seven hours and six minutes - the same amount as last year. TUC's figures show if everyone who works unpaid overtime did all their unpaid work at the start of the year, the first day they would get paid would be Friday 27 February.
The TUC traditionally declares this 'Work Your Proper Hours Day' and makes a light-hearted call for staff to work their proper hours for at least one day a year and for employers to thank their staff for regularly putting in the extra hours at work.
TUC general secretary Brendan Barber said: 'After years of progress, the numbers doing unpaid overtime has increased for the second year in a row. This is disappointing. But while some of this is due to the longs-hours culture that still dogs too many British workplaces, the recession will now be making many 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.' He added: 'This is not the year therefore for our usual light-hearted 'Work Your Proper Hours Day'.
But this does not mean people should ignore excessive working. Friday 27 February should still be used to think through working hours. Long hours are bad for people's health, and employers should never forget that each extra hour worked makes people less productive once they are over a sensible working week. 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.'
There is more information and some interactive applications available on the worksmart website here.
This information has been supplied by the TUC.