Brits clocked up a total of £25bn-worth of unpaid overtime in 2007, an increase on the year before and equivalent to almost £5,000 missing from each worker's pay packet.
Across the country, the number of employees working extra hours for nothing is close to five million, says the Trades Union Congress (TUC).
An extra 103,000 employees worked unpaid overtime in 2007 compared to 2006, bringing the total to almost one in five workers, according the TUC's analysis of government statistics. The average amount of unpaid overtime per person was slightly more than seven hours per week.
The regions where the most staff put in the most unpaid hours were the South East, where 890,000 clocked up extra time for nothing, and London (741,000).
Yorkshire & Humberside saw the steepest increase in numbers doing unpaid overtime. An extra 38,000 employees racked up unrewarded hours compared to last year, bringing its 2007 tally to 381,000.
The Eastern region of the country was not far behind, with an extra 37,000 workers subscribing to a long-hours culture, to make a total of 480,000 staff doing unpaid overtime.
The only regions where numbers fell were the East Midlands, where 57,000 fewer staff did free hard graft, and the North West and Merseyside, where numbers dropped by 41,000.
The regions where fewest staff worked unpaid overtime were Northern Ireland (74,000) and the North East (166,000).
The TUC said if workers in the UK who put in extra hours for nothing did all their unpaid work at the start of the year they would not be paid until 22 February — a date it has dubbed "Work Your Proper Hours Day". It added that employees who worked unpaid overtime last year missed out on an extra £4,955 in pay.
TUC general secretary Brendan Barber said workers in the UK are giving away too much for free to their employers, and should think how their time could be better spent with friends and family.