Teachers and lecturers do on average more unpaid overtime than any other occupation, a TUC poll suggests.
Lecturers and teachers do the most unpaid overtime
The TUC estimates that British workers lost £23bn in unpaid overtime in the past year, an average of £4,650 each.
On Friday, the TUC is urging staff to work only their contracted hours and it wants bosses to thank employees for extra work.
As part of its campaign, it has published a league table which allows workers to compare unpaid overtime.
The TUC based its findings on the Labour Force Survey.
The ongoing survey, conducted by National Statistics, asks 60,000 households each year about their working patterns, including how much unpaid overtime they do.
The calculations assume a normal working week of 40 hours and six weeks holiday a year.
According to the TUC's results, the average length of teachers' and lecturers' unpaid overtime a week is 11 hours 36 minutes, almost two hours more than the runners-up in the league table, senior managers.
Teachers, however, typically get 13 weeks holiday a year, a factor which has not been included in the research, while some lecturers can have 7 weeks or holiday or more.
According to the TUC, senior managers are losing the most money from working extra hours.
Their average unpaid overtime of nine hours and 48 minutes a week is worth £19,000 a year, the TUC said. Farm workers and arts workers also fare badly.
They work on average eight hours 54 minutes extra a week for about £2,500 a year on average.
Arts workers are also among the most underpaid and overworked. A typical arts worker will work an extra eight hours and six minutes a week, equal to £5,600 a year.
Among white collar workers, finance and accounts staff are the most overworked.
Those who do unpaid overtime put in seven hours 18 minutes a week on average, worth £6,000 a year.
The TUC's "Work Your Proper Hours Day" falls on 25 February.
It believes this is a day workers should celebrate.
"Too many workplaces are gripped by a long hours culture, where staff are expected to put in unpaid extra time week after week," said Brendan Barber, TUC general secretary.
"We are not saying that we should all become clockwatchers, but it's about time we called time on bosses who think the longer something takes the better the job is done."