Watching the World Cup on television could increase the risk of dying from heart disease, scientists warned.
Every hour a day spent sitting in front of a television increases the risk of death from heart disease by a further seven per cent of an individual's normal risk level, a study of 13,197 healthy middle-aged men and women in Norfolk has shown.
The Medical Research Council (MRC) research published in the International Journal of Epidemiology found 373 of the group - one in 35 - died from heart disease over a 10-year period.
The amount of time spent watching television was a "significant marker" of their likelihood of death from heart disease, the study found.
Researchers said an estimated eight per cent of these deaths, or 30 people, might have been avoided if TV viewing times had been reduced from the UK average of four hours a day to just one hour.
The calculation took into account other risk factors such as smoking, obesity, lack of exercise and poor diet.
Participants with a history of related diseases such as strokes and heart attacks were excluded from the study, and researchers measured television viewing time from questionnaires.
Katrien Wijndaele, co-author of the study, said there was a need for further research to see if other sedentary activities such as sitting behind a computer and car driving had the same effect.
She said: "Our bodies are not designed to sit for long periods and we should be aware that, as we put in the TV-hours watching the World Cup, our risk of heart disease is probably increasing. "It might seem obvious that watching TV is linked to heart disease but it's really crucial that we look closely at how our lifestyles affect our health in order to develop more effective ways of improving the health of the nation.
"This type of research is a crucial part of informing public health advice.
"We need further research to see if other sedentary activities, like sitting behind a computer or in the car, generate the same results.
"However, we chose to focus on TV as it's the most widespread sedentary leisure activity where people have an active choice to dramatically change their behaviour."
Ulf Ekelund, co-author of the study from the MRC, said: "If someone's normal risk of death by heart disease - taking into account other variables like lifestyle, gender and age - is 10 per cent, then just one hour a day of watching TV increases this risk by a factor of seven per cent to 10.7 per cent - a small but significant rise.
"So if my normal risk of dying by heart disease was 10 per cent and I also watched four hours of television a day, the national average, my risk would jump to 13 per cent.
"Substituting watching TV and sitting down for exercise such as brisk walking is the ultimate goal, but watching a couple of hours less TV a night and being physically active for at least 30 minutes a day can substantially reduce our risk of heart disease.
"In the future, doctors could use the number of hours spent in front of the TV as part of their assessment of our overall risk of heart disease."
Fotini Rozakeas, cardiac nurse at the British Heart Foundation, said: "This study strengthens the argument that sitting for hours on end watching TV is not good for your health. It's easy to snack on unhealthy foods and slouching on the sofa only burns a few calories.
"People can - and should - enjoy watching their favourite TV programmes or the World Cup without becoming couch potatoes by doing at least 30 minutes physical activity five times a week. This doesn't need to be a trip to the gym; a walk in the park or taking the stairs at work all help make a difference."