Workers are putting in billions of pounds worth of unpaid overtime, with the problem especially acute among teachers and managers, a new study shows.
The TUC urged companies to stop relying on staff doing extra hours for free and to make sure employees took a proper lunch break and left on time.
Research found that 5.3 million people worked an average of 7.7 hours a week in unpaid overtime last year, unchanged from the previous year, worth £33.6 billion.
Chief executives topped a list of those doing the most unpaid overtime, at an average of 13.2 hours a week, followed by teaching staff (12.1 hours), finance managers (11.3 hours) and managers in production and health care (10 hours).
The union organisation dubbed today Work Your Proper Hours Day, saying the average employee doing unpaid overtime had effectively worked for free so far this year.
The TUC warned that working time protections could be weakened when the UK left the EU.
General secretary Frances O'Grady said: "Few of us mind putting in some extra time when it's needed, but if it happens all the time and gets taken for granted, that's a problem.
"So make a stand today, take your full lunch break and go home on time.
"The best bosses understand that a long-hours culture doesn't get good results.
"So we're asking managers to set an example by leaving on time too.
"The Government still doesn't have a water-tight plan to stop working time protections getting weaker when we leave the EU.
"The Prime Minister should promise to put a guarantee into our future trade deals with Europe that British workers will have a level playing field with EU workers."
Workers in London did the most overtime, followed by those in the South East and South West.
Kevin Courtney, general secretary of the National Union of Teachers, said: "Once again, the TUC has found that teachers and the education sector as a whole are subject to enormous levels of unpaid overtime.
"This situation is untenable.
"Long and unmanageable working hours are the biggest single reason cited by teachers for leaving the profession."
University and College Union general secretary Sally Hunt said: "The time has come for schools, colleges and universities to recognise the hard work their staff do, reward them fairly and sort out their workloads."
A Department for Education spokesman said: "We are already reducing unnecessary work by working with teachers, the unions and Ofsted to challenge unhelpful practices and free teachers up to do what they do best - inspire all young people to fulfil their potential.
"We have taken action to address the key areas that teachers told us generate unnecessary work - marking, planning and data management, and we intend to take further steps to help address it through our action plan.
"We have also provided stability by pausing the introduction of new national tests until the 2018/19 academic year."