Every hour spent watching TV each day increases the risk of dying from heart disease by almost a fifth, say scientists.
Couch potatoes were warned that their lifestyle also increased the risk of death from other causes including cancer.
Individuals who spent hours watching television greatly heightened the chances of dying early from heart attacks and strokes, they found.
Compared with those watching less than two hours of TV, people who sat in front of the box for more than four hours a day were 80% more likely to die for reasons linked to heart and artery disease.
Researchers in Australia monitored 8,800 adults for six years to see what impact watching TV had on their long term health.
They found that each hour spent per day in front of the television increased the risk of death from all causes by 11%.
It also raised the risk of dying from cancer by 9% and the risk of heart disease-related death by 18%.
The scientists warned that it was not only telly addicts whose lazy lifestyles put them in danger. Any prolonged sedentary behaviour, such as sitting at an office desk or in front of a computer, posed similar risks, they said.
It also made no difference whether or not a person was overweight or obese.
"Even if someone has a healthy body weight, sitting for long periods of time still has an unhealthy influence on their blood sugar and blood fats," said lead researcher Professor David Dunstan, from the Baker IDI Heart and Diabetes Institute in Victoria, Australia.
The average amount of TV people watch each day is three hours in both Australia and the UK, said the scientists.
In the US, where two-thirds of adults are overweight or obese, some people spent as much as eight hours watching television - the equivalent of a nine to five working day.
"What has happened is that a lot of the normal activities of daily living that involved standing up and moving the muscles in the body have been converted to sitting," said Prof Dunstan. "Technological, social, and economic changes mean that people don't move their muscles as much as they used to - consequently the levels of energy expenditure as people go about their lives continue to shrink. For many people, on a daily basis they simply shift from one chair to another - from the chair in the car to the chair in the office to the chair in front of the television."
The scientists interviewed 3,846 men and 4,954 women aged 25 and older who underwent sugar tolerance tests and provided blood samples.
Participants were recruited from 1999 and studied for the next six years. Based on their own reports of TV viewing they were grouped according to whether they watched less than two hours a day, between two and four hours, or more than four hours.
During the follow-up period there were 284 deaths, 87 due to cardiovascular, or heart and artery disease, and 125 from cancer.
While the association between cancer and television viewing was only modest, there was a strong link between TV watching and a higher risk of cardiovascular death. This was despite taking account of recognised heart disease risk factors such as raised cholesterol levels and other lifestyle factors.
The findings were reported today in Circulation: Journal of the American Heart Association.
Prof Dunstan had this message for members of the public: "In addition to doing regular exercise, avoid sitting for prolonged periods and keep in mind to 'move more, more often'. Too much sitting is bad for health."