UK employees who did unpaid overtime in 2004 would have each earned £4,650 for their efforts if they had received a wage, a new report from the TUC says.
The union body said its research indicated many firms now relied on staff putting in extra hours.
Data showed workers across the UK put in a total of £23bn in unpaid overtime.
The TUC said the average person would have worked for free until 25 February last year, had all their unpaid work taken place at the start of the year.
It has dubbed 25 February 2005 “Work Your Proper Hours Day”.
It is urging employees on that day only work their contracted hours to remind their bosses how much they depend on the unpaid extra work and loyalty of their staff.
TUC general secretary Brendan Barber said people are putting in long hours to make up for poor organisation.
“Most people do not mind putting in some extra time when there’s a crisis or an unexpected rush,” he said.
“But too many workplaces have come to depend on very long hours. They get taken for granted.”
The reports showed workers in London put the most unpaid overtime – about eight extra hours every week – followed by those in Wales and the West Midlands.
“It also puts employer complaints about the costs of benefits such as pensions or time off for new parents into perspective,” Mr Barber added.
“Employers have been cutting back on pensions even as their staff put in longer hours.”