The TUC says workers are missing out on an average £5,000 in pay
The economic downturn is leading to workers putting in record levels of unpaid overtime, the TUC has said.
It estimates 5.24 million people put in extra work worth £26.9bn in 2008.
TUC general secretary Brendan Barber said while some of the rise was down to a "long-hours culture", workers' fears of losing their jobs was also a factor.
"Inevitably, people will be putting in extra hours if they think it can help protect against redundancy or keep their employer in business," he said.
According to the TUC the average amount of unpaid overtime was more than seven hours a week, and workers were missing out on an average of £5,000 of pay.
Mr 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 long-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."
The areas of the country that saw the biggest rises in unpaid overtime were London, the East Midlands and eastern England, said the TUC.
Mr Barber added: "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."