Skateboarding is the kind of activity the BHF wants to encourage
Only one in eight youngsters is getting the recommended 60 minutes of physical activity a day, according to the British Heart Foundation (BHF).
It surveyed more than 1,000 children aged eight to 15 in July and August.
The survey found that a third of the children did less than an hour of exercise a week and 20% thought you only needed to do it if you were fat.
Researchers say the lack of activity is starting at a younger age and energy- dense snacks compound the problem.
The BHF has launched a campaign aimed at obese 11-13-year-olds.
The Food4Thought campaign is designed to encourage children to consider their levels of physical activity and the long term consequences of their current food choices.
Recent research has predicted that, if current trends continue, two thirds of all children will be overweight or obese by 2050.
Dr Mike Knapton, director of prevention and care at the BHF said: "We have a generation of kids growing up who have a shockingly blase attitude towards exercise and being active.
"Young people need to switch off their square eyes and get in the habit of exercising now."
Former Olympic champion Sally Gunnell is backing the campaign. She said: "Children's sluggish attitude towards daily exercise is worrying and it's great to see the BHF thinking of new ways to get them intrigued and active again."
Dr David Wilson, senior lecturer in Paediatric Gastroenterology and Nutrition at the University of Edinburgh, agreed and said the lack of physical activity was starting at a younger age.
He said: "Sedentary behaviour is a very worrying problem - the problem is starting in nursery school.
"But obesity is absolutely about both food intake and activity - they are doing nothing and eating more.
"The energy-dense snacks they consume in front of their televisions and computers are simply not being burnt off.
"There has been a two to three times increase in obesity in the last 15 years and there are signs from around the world that the numbers are stabilising but the small number of super obese is continuing to increase."
Dr Ian Campbell, of the charity Weight Concern, said: "Children need to be active to prevent obesity but also to promote healthy growth, bone strength and psychological wellbeing.
"It is important kids know how physical activity can help them but equally important that their parents understand that too.
"Good habits and skills learned now can ensure children can have a lifetime of physical activity, the benefits of which will stay with them throughout and enable them to live healthier and happier lives."