The trade union body has calculated that managers in advertising, public relations and marketing put in an average of 9.3 unpaid hours every week, which means that they work for their companies for free until March 9.
Less senior marketing staff on average do 6.8 hours unpaid overtime a week, meaning that they start to get paid on Feb 22.
The TUC has declared Friday to be Work Your Proper Hours Day and is encouraging people who work unpaid overtime to exercise their right to work no more than their contracted hours and remind their bosses just how much modern workplaces depend on unpaid overtime.
The report into unpaid overtime is part of the TUC's "It's about time" campaign against the UK's long-hours culture and for a better work/life balance.
The TUC is calling for marketers to work their proper hours for one day a year.
Brendan Barber, general secretary of the TUC, said: "We're not calling on Britain to turn into a nation of clock-watchers 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."
He added: "Marketing is a long-hours world and that's why we're saying that for one day a year people should work their proper hours. They should come in on time, take their proper lunch break and leave when they are meant to, preferably on their way to somewhere where their boss will buy them a coffee or a cocktail."
The TUC has calculated the figures based on the September 2003 Labour Force Survey, showing the mean hours of overtime worked by those in each occupational group who do more than one hour unpaid overtime a week. The day in the year when those who do unpaid overtime start to get paid is calculated by assuming a standard working week of 40 hours.
It found that chief executive officers and senior civil servants work the most unpaid overtime, followed by teachers, farmers and health professionals. A full list of professions can be found here.
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