British workers are giving away the equivalent of �23bn of free work every year through unpaid overtime, according to research from the TUC.
More than five million workers do unpaid overtime work beyond their contracted hours, the body said.
Top of the list are senior civil servants, teachers, farm managers and health professionals such as doctors.
As part of a TUC campaign to highlight long hours, a new league table allows workers to compare unpaid overtime. Proper hours
The league table has been released in the run up to the TUC's "Work Your Proper Hours Day" on 27 February.
Teachers work more unpaid hours than doctors
Farm staff do 39% less "free" overtime than their bosses
Plumbers work more unpaid hours than estate agents
According to the TUC, this is the date when the average UK worker who does unpaid overtime finishes the 40 unpaid days they do every year and starts earning for themselves.
However, some workers have much longer to work before they start working for themselves. Top civil servants, the group that do the longest unpaid overtime, must wait until 27 March. Teachers, who are the second worst-off in the unpaid overtime stakes, must wait until 24 March.
The side-effects of long hours, such as stress and ill-health, can be very damaging for workers. Long hours can also wreck relationships and make caring for children more difficult, the TUC said. It wants workers to vote with their feet on 27 February and work only their proper contracted hours. "We're not calling on Britain to turn into a nation of clock-watchers," said TUC general secretary Brendan Barber.
"But too many of Britain's bosses who depend on the unpaid overtime of their staff take it for granted and never show their appreciation." Work slaves
Britain's love affair with long hours is not necessarily paying dividends, and going home on time could be good for the economy.
It is an often cited statistic that workers in the UK work the longest hours in Europe. However, British workers are not as productive as employees in other countries, when assessed on output per hour.
In France, many employees work a maximum 35-hour week but achieve a higher rate of productivity. Recently released figures from the Office for National Statistics show France is 31.7% and Germany 16.4% more productive by hour than Britain. BBC News