Research by the TUC found that almost five million people were working an average of over seven hours a week without pay.
The amount of free time put in by workers last year was worth over £31 billion, according to a new study.
Almost five million people were working an average of over seven hours a week without pay, according to the research by the TUC.
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."
How much overtime do you work?
If you think you're going well above your weekly threshold, calculate your hours below to work out how much you could be losing in earnings.
Working hours: What your rights are
The government's Working Time Regulations determine the maximum weekly working time, patterns of work and holidays, plus the daily and weekly rest periods for workers in Britain.
They also cover the health and working hours of night workers.
The Regulations apply to both part time or full-time workers, including the majority of agency workers and freelancers.
The Advisory, Conciliation and Arbitration Service (Acas) says the Working Time Regulations provide rights to:
A limit of an average 48 hours a week on the hours a worker can be required to work, though individuals may choose to work longer by "opting out".
Paid annual leave of 5.6 weeks' a year
11 consecutive hours' rest in any 24-hour period
A 20-minute rest break if the working day is longer than six hours
One day off each week
A limit on the normal working hours of night workers to an average eight hours in any 24-hour period, and an entitlement for night workers to receive regular health assessments.
There are special regulations for young workers, which restrict their working hours to eight hours per day and 40 hours per week. The rest break is 30 minutes if their work lasts more than 4.5 hours. They are also entitled to two days off each week.
Help - 'I'm being forced to work more than 48 hours'
Your employer can't force you to work more than 48 hours a week if you haven’t agreed to it in writing. If they do, they could be breaking the law.
If you feel they're taking advantage, raise your concern with your employer to try and work it out.
The Citizens Advice Bureau (CAB) says if you want to make a formal complaint, you are well within your rights to do so.
To do this, put it in writing to your company or boss. The letter should set out what your complaint is with enough detail for your employer to be able to investigate it properly.
You can use CAB's template letter to set out the details of your grievance . https://www.citizensadvice.org.uk/work/ ... e-at-work/" onclick="window.open(this.href);return false;
If you need to take matters further, you can make a claim to an employment tribunal .
To do this, you must notify the Advisory, Conciliation and Arbitration Service (Acas) of your potential claim first.
If you refuse to work more than 48 hours a week and feel like you're being treated differently because of it, you could be being unfairly treated. If nothing is resolved, you can raise a grievance and then go to a tribunal.