Children who spend hours in front of the television are at greater risk of developing asthma than those who are more active, a study has found.

Youngsters who watched more than two hours of TV a day were twice as likely to get the respiratory condition as children who watched less, according to research by British scientists.

The greater risk of asthma was not directly caused by watching television, which was used only as an indicator of how sedentary the children's lifestyles were.

The finding builds on recent work that points to a link between asthma and low levels of physical activity. Some scientists believe that inactive children do not inhale deeply and regularly enough, which helps to stretch the airways and may make them less prone to asthma.

The condition develops when small airways called bronchioles become inflamed and swollen, restricting how much air can enter and leave the lungs. More than five million people in Britain take asthma medications, including around a million children.

The study by researchers at Glasgow University drew on the medical records of 14,000 children who were followed from birth until the age of eleven-and-a-half years. Throughout, the parents were asked whether their children had shown signs of wheezing, and if their GP had diagnosed asthma.

Andrea Sherrif and her team found that overall 6% of children whose breathing was healthy at 39 months went on to be diagnosed with asthma by the age of eleven-and-a-half. But children who watched more than two hours of TV a day at 39 months were twice as likely to have developed the condition. The study appears today in the journal Thorax.

Doing little exercise caused an equal increase in asthma in boys and girls, and was not related to their weight.

Elaine Vickers at the charity Asthma UK said: "The findings add to a wealth of evidence linking a lack of exercise and being overweight with an increased risk of asthma, but this study is the first to directly link sedentary behaviour at a very young age to a higher risk of asthma later in childhood.

"We have one of the highest rates of childhood asthma in the world so it is especially important that parents in the UK try to prise their kids away from the TV and encourage them to lead an active lifestyle. This includes children with asthma, who can also greatly benefit from regular exercise."