Researchers found the average employee puts on just under half a stone in their first year after changing roles.
See also: The worst excuses for skiving off work
One fifth blame their colleagues for their expanding waistlines, while one in 10 claim the boss is responsible.
Six in ten admit their sedentary lifestyle – sitting at a desk for eight hours a day – has contributed to their weight gain.
One fifth blame their colleagues for their expanding waistlines
More than half said they find it impossible to resist biscuits, sweets and cakes brought in by colleagues.
And one in six said the only way they would have any success with a new diet would be if fellow workers joined them on their health quest.
Dr Emma Derbyshire, a registered Public Health Nutritionist speaking on behalf of Seasonal Berries said: "When we are at work and energy levels are low there is a tendency to reach for biscuits or chocolate for a quick energy rush.
Being too tired to exercise after work is another reason people get fatter
"However, our blood sugar levels will soon plunge back. Opting for fruits that are naturally sweet such as blueberries or raspberries that are easy to eat in the office will do us far more good in the long run.
"These are a natural source of energy without the free sugars of biscuits or chocolate whilst also providing a good dose of antioxidants and fibre".
Birthday cakes in the office, free lunches or breakfasts from employers and a vending machine in the office are all held responsible for office workers gaining weight.
54 per cent will eat unhealthily if they're feeling stressed
While 17 per cent of people polled say working near to a fast food restaurant doesn't help their bid to maintain a healthy lifestyle.
And six in 10 find it impossible to resist naughty food when they're being offered around the office.
The findings show pressures at work also lead workers to reach for treats such as chocolate, biscuits or cakes – with 34 per cent admitting they're more likely to give into temptation in on a busy day, while 54 per cent will eat unhealthily if they're feeling stressed.
26 per cent don't watch what they're eating if they're bored
That 'Friday feeling' leads 27 per cent to eat badly, while 26 per cent don't watch what they're eating if they're bored.
The study shows the average office worker reaches for treats just three hours into the normal working day.
It may come as no surprise then, to learn that over the course of one year, office workers have tried and failed on FIVE diets – and four of these failures are a direct result of temptations at work.
Almost a quarter of workers say when they do diet, their colleagues don't encourage them, and 38 per cent say keeping trim is hard when you have a boss who is overly generous with treats.
More than half said they find it impossible to resist biscuits, sweets and cakes
Dr Emma Derbyshire for Seasonal Berries, who carried out the study via OnePoll.com added: "Office lifestyles can be tough on our waistlines.
"Drinking water, keeping a daily log of foods and drinks eaten and having some healthy snacks to hand such as vegetable sticks, nuts or berries can all help.
"Whether in schools, parties or in the office when bringing in treats we shouldn't immediately turn to biscuits, chocolate and sweets.
''A punnet of berries can be just as tasty without contributing to excess calories".
The top 10 reasons for weight gain at work:
- Sitting at the desk for a long time
- Stress / having a busy day at work
- Feeling tired
- Being too tired to exercise
- Too many offers of free cake and biscuits from colleagues
- Not having time to exercise
- Bad weather outside
- Long hours resulting in late or unusual meal times
- Always buying lunch for work instead of homemade lunches