Watching television for more than six hours a day is more hazardous than smoking, according to a survey.
Every hour spent in front of the box after the age of 25 slashes life expectancy by 22 minutes compared to those with a more active pastimes.
The survey, carried out by Australian researchers on 11,000 adults, revealed a sedentary lifestyle is as bad for people as cigarettes and obesity.
Not only does it increase levels of inactivity, but watching TV also poses more opportunities for unhealthy eating.
Writing in the British Journal of Sports Medicine, the academics, from the University of Queensland, said: "TV viewing time may be associated with a loss of life that is comparable to other major chronic disease risk factors such as physical inactivity and obesity."
They used information from the Australian Diabetes, Obesity and Lifestyle Study, together with population and death rate data.
But they said: “While we used Australian data, the effects in other industrialised and developing countries are likely to be comparable, given the typically large amounts of time spent watching TV and similarities in disease patterns.”
Adults spend on average four hours a day watching TV compared with five hours in the US.
The findings were also compared to smoking, with an average of 11 minutes of life lost to every cigarette.
England's Chief Medical Officer, Sally Davies, said: "Physical activity offers huge benefits and these studies back what we already know – that doing a little bit of physical activity each day brings health benefits and a sedentary lifestyle carries additional risks.
"Adults, for example, can get their 150 minutes of activity a week in sessions of 10 minutes or more and for the first time we have provided guidelines on reducing sedentary time.”