The amount of free overtime put in by workers last year was worth over £31 billion, according to a new study.
Research by the TUC found that almost five million people were working an average of over seven hours a week without pay.
The extra work was worth an average of £6,265 per worker, it was estimated.
The TUC calculated that the average person has effectively worked for free so far this year, only starting to be paid from Friday.
The union organisation urged employees to take a proper lunch break on Friday and to leave on time.
TUC general secretary Frances O'Grady said: "Lots of us are willing to put in a bit of extra time when it's needed, but it's a problem if it happens all the time. So today we're saying to workers, make sure you take a proper lunch break and go home on time.
"We're asking managers to leave on time too. Good bosses know that a long-hours culture doesn't get good results, and the best way to lead is by example."
The TUC analysis also found that while public sector workers make up a quarter of all employees, they account for more than a third of all unpaid overtime.
Unison's assistant general secretary Christina McAnea, commented: "Public sector staff regularly work through their breaks and go home late, because they're dedicated to the patients, students and local people who rely on them.
"But expecting employees to work above and beyond the call of duty, day in day out, is simply not on.
"With staffing shortages and wages failing to rise with the cost of living, morale in our public services is already at rock bottom. Overstretched and under-appreciated staff are at risk of burning out or giving up on a career in the public sector altogether."