Millions of workers put in £31.5 billion of unpaid overtime last year, a new study has shown.
More than five million people worked an average of 7.7 extra hours a week without pay, worth over £6,000 each, said the TUC.
Workers aged 40 to 44 were most likely to do unpaid overtime, with teachers doing the most (an average of 11.9 hours a week), followed by financial institution managers (11.2), production managers (10.3) and health and care service managers (9.9).
The TUC has dubbed Friday Work Your Proper Hours Day, to mark the time when the average worker putting in unpaid overtime would start getting paid if they worked all unpaid hours from the start of the year.
General secretary Frances O'Grady said: "Too many workplaces tolerate a long-hours culture. That is why we are calling on employees to take a stand today on Work Your Proper Hours Day and take a full lunch break and go home on time.
"We do not want to turn Britain into a nation of clock watchers. Few people mind putting in extra effort from time to time when it is needed, but it is too easy for extra time to be taken for granted and expected day in day out."
The number of workers doing unpaid overtime fell slightly last year compared to 2014, but was more than in 2001, said the TUC.
Ann Francke, chief executive of the Chartered Management Institute, said: "Most of us are comfortable with the idea that a modern workplace requires us to occasionally pitch in out of hours.
"But the 'always on' culture must be switched off, with line managers encouraged to support an 'always willing' mindset that reflects the give and take necessary for a higher quality of working life."
Unison general secretary Dave Prentis said: "Spending cuts have meant redundancies and recruitment freezes across the public sector. This means those who remain are overstretched and spread ever more thinly, as they valiantly try to cover the work of colleagues who have either been made redundant or not replaced.
"It will come as no surprise to council, NHS, school or police staff that they are contributing more than their fair share of unpaid overtime. The recent NHS staff survey showed that nearly three quarters of health workers regularly work beyond their contracted hours for free.
"This is no way to run public services, and with no respite in the cuts, staff will continue to bear the brunt."