Researchers found a typical member of staff puts in an extra 305 hours and 30 minutes each year - often unpaid and unappreciated.
The study of 2,000 working adults shows on average people start work at least 17 minutes before they're officially meant to.
They only take 31 minutes of their hour lunch break and leave the building 16 minutes later than they should.
In addition, people will spend a further 16 minutes at home sifting through emails, before and after work.
This hard work adds up to to overtime of one hour and 18 minutes every day - six hours and 30 minutes each week.
When asked why they work above and beyond the call of duty, 41 per cent say they feel it's the only way they can do their job properly.
Six in 10 people claim they are EXPECTED to work additional hours and put in overtime as and when necessary, with only five per cent of these people enjoying more money as a result.
While 33 per cent of employees are made to feel guilty if they leave the office first at the end of the day.
The study was carried out by SPANA, a veterinary charity, as part of a campaign to highlight the plight of overworked animals and compare their lives with those of working humans.
Chief executive Jeremy Hulme said: "I think most people will be surprised to learn that British employees work the equivalent of seven and a half weeks extra each year without any more money to show for it.
"Thankfully the majority of people enjoy their jobs and have reasonable working conditions - unlike the millions of working animals overseas.
"These animals can work for 12 or more hours a day, pulling backbreaking loads with few breaks and little food and water. For these hardworking and forgotten animals, there's no pay, no health and safety protection and no retirement to look forward to.
"We believe that a life of work shouldn't mean a life of suffering - these vital animals deserve our protection and care."
However, the study shows there are some positive aspects about the long hours worked by staff in Britain.
Forty five per cent are confident that their boss can differentiate between those workers who only work the hours they are paid, and those who always do extra.
And, happily, six in 10 workers believe their management do appreciate the amount of work they put in, and two thirds feel the work they put in is worthwhile.
When it comes to key reasons why Britain's workers are prepared to work longer than they are paid for, a quarter admit they can't manage their workload on their current hours and a fifth suffer because of staffing shortages at work.
Other reasons cited for working a little extra include wanting to be the best at the job, loving the work, and feeling the need to impress management.
However, putting in extra hours does take its toll - and 54 per cent of people admit they are often absolutely exhausted by the end of the working week.
And a fifth of people feel at times they need a complete overhaul of their work life balance.
SPANA, provides free veterinary treatment to working animals in developing countries throughout the world.
Working animals, such as horses and donkeys, do the jobs of tractors, trucks and taxis and are relied upon entirely by the world's poorest people for their livelihoods.
These animals work long hours in difficult conditions and never get to enjoy holidays or retirement.
TOP 10 REASONS FOR WORKING UNPAID OVERTIME
1. It is the only way I can get my job done properly
2. I cannot manage my workload on my contracted hours
3. We are understaffed at work so I have no choice
4. I want to be the best at what I do, and that requires a little extra effort
5. I love my job
6. I feel I need to impress my boss / management
7. I feel my job is safer if I go above and beyond the call of duty
8. I find it hard to switch off from work when I leave the office
9. I'm trying to work my way up the career ladder and this is the way to do it
10. Morale is good at work so we're all quite keen to work longer hours
Spend four minutes at home checking emails
Arrive 17 minutes before contracted time
Work through 29 minutes of lunch
Leave 16 minutes after contracted time
Spend 12 minutes working on emails in the evening
TOTAL OVERTIME PER DAY - 78 MINUTES / 1 HOUR 18 MINUTES
TOTAL OVERTIME PER WEEK - 390 MINUTES / SIX HOURS 30 MINUTES
TOTAL OVERTIME PER YEAR - 390 X 47 WEEKS = 18,330 / 305 HOURS AND 30 MINUTES
Across an average 40 hour week (Monday to Friday 9-6pm with an hour's lunch) - ADDITIONAL 38 WORKING DAYS A YEAR