A couch potato lifestyle is linked to a significantly greater risk of certain cancers as well as heart disease and diabetes, research has shown.
Every extra two hours spent sitting each day was associated with a 10% increased chance of developing cancer of the womb lining in women, while bowel and lung cancer risk was raised by 8% and 6% respectively.
The effect appeared to be unrelated to how much exercise people took when not sitting. This suggests that even in people who are generally physically active, sitting down for too long can increase cancer risk.
Scientists came to the conclusion after studying pooled data from 43 studies with more than four million participants and almost 70,000 cancer cases.
This kind of research, known as "meta-analysis", can uncover trends that might be hidden from individual studies recruiting small numbers of people.
All the studies analysed involved questionnaires and interviews probing lifestyle habits related to activity such as TV viewing time, sitting time at home and at work, and total sitting time.
Comparing the highest and lowest levels of sedentary behaviour revealed a statistically significant increased risk for three specific cancers - bowel, endometrial (womb lining) and lung - associated with sitting.
Study authors Daniela Schmid and Dr Michael Leitzmann, from the University of Regensburg in Germany, wrote in the Journal of the National Cancer Institute: "That sedentariness has a detrimental impact on cancer even among physically active persons implies that limiting the time spent sedentary may play an important role in preventing cancer."
TV viewing time was most strongly associated with bowel and endometrial cancers - possibly because watching TV is often accompanied by eating junk food and drinking sweetened beverages, said the researchers.
The scientists pointed out that the link between sitting time and lung cancer was only marginally statistically significant.
In the studies analysed, the least amount of time people spent sitting down was about two or three hours.
Each two hours per day increase in sitting time above this level was said to increase the risk of bowel, endometrial and lung cancer.
No association was seen between sedentary behaviour and the risk of breast, ovarian, prostate, stomach, oesophageal (gullet), testicular, kidney and rectal cancers.