More than half of parents believe their children spend too much time sitting down because they are on mobile phones or games consoles, a survey has suggested.
Internet Matters, a non-profit organisation which helps families stay safe online, conducted a study ahead of the autumn half-term break.
It indicated 54% of parents were worried that screen time was fuelling their child's inactive lifestyle, with more than a third (36%) saying device use meant their children did not play outside enough.
Parents were asked about their children's use of any connected device, which could be a mobile phone, laptop or tablet.
The survey of 2,022 parents in the UK indicated they worry more about exercise as their children get older.
Nearly half (49%) of parents with children aged 14 to 16 were concerned about a lack of exercise, compared with 31% of those with youngsters aged four to five.
The NHS has warned that a "sedentary lifestyle" can increase the risk of developing health problems such as diabetes, weight gain and obesity.
Cancer Research UK said this week that children who spent time online are more likely to pester their parents for junk food, due to exposure to adverts for unhealthy foods.
Overall, the research showed 47% of parents were concerned their child spends too much time online.
The survey found that four out of 10 parents said screen time was disturbing their children's sleep and 35% said it affected time spent together as a family.
Carolyn Bunting, chief executive of Internet Matters, said: "Parents should not be afraid to have limits for screen time and set boundaries over their child's internet use.
"The amount of time that children should spend online is a big issue for parents - and many will worry that too long watching screens may have a negative impact on their physical and mental health."
She said there apps available that encourage children to be active, which "can be a great opportunity to share and learn about tech in a positive way and use it as a force for good".