Kids aged four who have a telly in their bedroom are more likely to grow up to be fat teenagers, warns a new study.

Those allowed the treat before they start school are more likely to be obese and anti social by the time they reach their teenage years, according to the research.

They are also prone to being unhappy and depressed - as well as violence and being bullied, suggests the study.

The findings are based on almost 2,000 youngsters tracked for nearly a decade - and will also apply to tablets or smartphones, say the Canadian team.

Too much screen time could be consigning little ones to a lifetime of illness, they warned.

It deprives a child of activities that are more enriching for its development during the vital pre-school years.

Lead author Professor Linda Pagani, of the University of Montreal, saidd: "The location of the TV seems to matter. Having private access to screen time in the bedroom during the pre-school years does not bode well for long-term health.

"The children in our study were born at a time when television was the only screen in the bedroom.

"Today, given the portability of digital devices and the constant switching from one device to another, the guidelines of the American Academy of Pediatrics clearly have reason to encourage screen-free zones and screen-free locations at home, especially given the implications for the growth and development of children.

"Our research supports a strong stance for parental guidelines on the availability and accessibility of TVs and other devices."

The study, published in the journal Pediatric Research, found those with a TV in the bedroom at four were more at risk of being significantly overweight by the age of 13.

Unhealthy eating habits were more prevalent along with lower levels of sociability and higher rates of emotional distress.

Depressive symptoms, victimisation and physical aggression were also more common - regardless of potential individual and family factors predisposing them to problems.

Educational psychologist Prof Pagani said: "The early years are a critical period in a child's development.

"Intuitively, parents know how their children spend their leisure time will impact their well-being over the long term.

"And with TV being their most common pastime, it is clear the many hours they spend in front of the screen is having an effect on their growth and development, especially if the TV is in a private place like the bedroom."

With their attention diverted, children risk not having enough physical and social interactions to promote proper physical and socio-emotional development.

Prof Pagani said: "To test that hypothesis, we longitudinally followed a birth cohort to examine whether there was a link between having a bedroom TV at age 4, during the neurodevelopmentally critical preschool period, and later physical, mental, and social problems in early adolescence.

"Our goal was to eliminate any pre-existing conditions the children or families had that could bias our results."

Her team followed more than 1,800 children born in Quebec between the springs of 1997 and 1998 with independent examiners assessing their health at 13, including measuring their BMI (body mass index).

The adolescents also reported their consumption of unhealthy foods. To measure psychological problems, teachers rated how much emotional stress the children faced.

They also completed a short version of the Children's Depression Inventory - a psychological questionnaire used to identify the mental illness.

For social problems, teachers also reported on how the kids got along with their peers and whether they were bullied.

Prof Pagani, who presented the research at the International Convention of Psychological Science in Paris, said all these measures are good predictors of later physical and mental health in adulthood.

Two years ago a study of more than 12,000 young children in the UK also showed children who have TVs in their bedrooms are more likely to be overweight than those who do not.

For girls in particular, the University College London team found the longer spent watching TV, the more likely they were to put on weight.

The scientists found more than half the children had TVs in their bedrooms at the age of seven.

Girls were 30% more likely to be overweight when they were 11. For boys, the risk was increased by about 20%.

The researchers suggested it may be down to children getting less sleep when watching TV in their bedrooms, or snacking in front of their screens.

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, they said.

Experts are calling for strategies designed to prevent childhood obesity to do more to tackle this issue.

They say as "our screens have become flatter, our children have become fatter."