Children who have TVs in their bedrooms are more likely to be overweight than those who do not, a study by University College London scientists suggests.
For girls in particular, they found the longer spent watching TV, the more likely children were to put on weight.
Researchers say there is now an urgent need to see if similar patterns exist with laptops and mobile phones.
Experts said high levels of screen time exposed children to a damaging combination of risks to health.
'Watch and weight'
Published in the International Journal of Obesity, the study analysed data from more than 12,000 young children in the UK.
Scientists found more than half the children had TVs in their bedrooms at the age of seven.
Parents were also asked to rate how many hours a day children generally spent watching TV.
Later, when the children were 11, researchers plotted their body mass index (a ratio of height and weight) and looked at the percentage of body fat.
Girls who had TVs in their bedrooms at the age of seven were 30% more likely to be overweight when they were 11, compared to children who did not have TVs in their bedrooms.
For boys, the risk was increased by about 20%.
Researcher Dr Anja Heilmann, said: "Our study shows there is clear link between having a TV in the bedroom as a young child and being overweight a few years later."
Researchers say they cannot be sure why the link between TVs and being overweight exists, but suggest it may be down to children getting less sleep when watching TV in their bedrooms or snacking in front of their screens.
And they hypothesise that the stronger link between the hours girls spend watching TV and being overweight could be influenced by girls being less likely to be physically active than boys at this age.
Researchers are calling for strategies designed to prevent childhood obesity to do more to tackle this issue.
Writing in the journal, they say: "While our screens have become flatter, our children have become fatter."
Prof Nick Finer, consultant endocrinologist and bariatric physician at University College London, said the study was "powerful" although it couldn't prove that a bedroom TV directly caused weight gain.
But he added: "It is hard not to think that parents concerned about their child's risk of becoming overweight might appropriately consider not putting a TV in their young children's bedrooms."
Poor eating habits
Prof Russell Viner, from the Royal College of Paediatrics and Child Health (RCPCH) said the findings should be taken seriously.
"With a third of 11-year-old children in England overweight and almost one in five obese, urgently tackling the childhood obesity epidemic is absolutely vital.
"We know that high levels of screen time expose children to increased risks of being overweight on a number of fronts, creating a damaging combination of a more sedentary lifestyle, increased exposure to junk food advertising, disruption to sleep and poorer ability to regulate eating habits when watching TV."
Prof Viner said the study supported their call for a ban on junk food advertising on TV before the 21:00 watershed.