UK employees did unpaid overtime worth £23 billion last year, according to an analysis of latest figures by the TUC.
On average each employee who did unpaid overtime would have earned £4,650 for their unpaid hours if they had been paid at their normal hourly rate, it estimated.
If they had done all their unpaid overtime at the beginning of the year, they would have worked for free until Friday 25 February.
The TUC has as a result called for a “Work Your Proper Hours Day” this year, to take place on 25 February.
This is the day once a year that the TUC urges employees only to work their contracted hours to remind their bosses how much they depend on the unpaid extra work and loyalty of their staff.
Bosses should take their staff out for a lunchtime or after-work meal, coffee or cocktail to say thank you, recommended the TUC.
TUC general secretary Brendan Barber said: “We’re not saying that we should turn into a nation of clock-watchers. Most people do not mind putting in some extra time when there’s a crisis or an unexpected rush.
“But too many workplaces have come to depend on very long hours. They get taken for granted and staff have to do even more if there is an unexpected rush,” he added.
Many long hours workplaces were simply inefficient and unproductive, with people putting in long hours to make up for poor organisation or planning, he suggested.
“It also puts employer complaints of the costs of benefits such as pensions or time off for new parents into perspective. Employers have been cutting back on pensions even as their staff put in longer hours,” he said.
Londoners put in the longest hours, with an extra seven hours 54 minutes in a week - almost a full extra eight-hour day.
If paid for this they would have earned an extra £7,000 a year. They were followed by employees in Wales (seven hours 42 minutes, or £4,320) and West Midlands (seven hours 36 minutes, or £4,410).