A survey has revealed many parents believe their children to be unfit, but more than a third admitted they had no interest in improving their fitness.
Around half of the 1,000 parents surveyed believed their sons and daughters could not run 3km (1.9 miles) without stopping, while 41% said their children had no interest in taking part in any sport.
When asked what they thought was to blame, parents listed the use of social media, mobile phones, computer games and an inability to access sporting facilities.
The research was commissioned by the Royal Parks Foundation ahead of Pulse3K - a 3km race aimed at 11 to 16-year-olds in Hyde Park on October 10.
Personal trainer Matt Roberts said the average teenager should be able to run a distance of 3km without too much difficulty.
"Exercise makes children brighter, more energised and more responsive," he said. "The fact that so many children are unwilling to do something that should come naturally to them is a sad reflection of the way that parents currently encourage, motivate and inspire their children."