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Babies and toddlers should not be left to passively watch TV or other screens, according to new World Health Organization guidelines.

Sedentary screen time, including computer games, should not happen before a child is two, the WHO says.

The limit for two- to four-year-olds is an hour a day and less is better.

The UK has no plans to update its own advice on screen use, which sets no time limits, although it says children should avoid screens before bedtime.

The UK's Royal College of Paediatrics and Child Health insists there is little evidence screen use for children is harmful in itself.

The new WHO advice focuses on passive viewing - youngsters being placed in front of a TV or computer screen or handed a tablet or mobile phone for entertainment - and is aimed at tackling child inactivity, a leading risk factor for global mortality and obesity-related ill health.

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It is the first time the WHO has made recommendations on physical activity, sedentary behaviour and sleep for children under five.

As well as warning against passive screen time, it says babies should not spend longer than an hour at a time strapped into a buggy, car seat or sling.

The guidelines will be presented at the European Congress on Obesity in Glasgow on Sunday.

The advice

For babies:

Be physically active several times a day, including at least 30 minutes' "tummy time" - lying on their front

No sedentary screen time

14-17 hours' sleep a day, including naps, for newborns - reducing to 12-16 by four to 11 months

Should not be restrained (ie strapped into a recliner, seat or sling) for more than an hour at a time

For one- and two-year-olds:

At least three hours' physical activity a day

No sedentary screen time for one-year-olds and less than an hour for two-year-olds

11-14 hours' sleep a day, including naps

Should not be restrained for more than an hour at a time or sit for extended periods of time

For three- and four-year-olds:

At least three hours' physical activity a day, including at least one of moderate or vigorous intensity

Up to an hour of sedentary screen time - less is better

10-13 hours' sleep a day, which may include a nap

Should not be restrained for more than an hour at a time or sit for extended periods of time

The WHO advice is based on available evidence, but there is still a lack of definitive research into the harms and possible benefits of screen use.

However, it was unlikely very young children gained from passive, sedentary viewing, said one of the guideline authors, Dr Juana Willumsen.

"Sedentary time should be made into quality time. Reading a book with your child, for example, can help them develop their language skills.

"A child who is given a tablet to keep them quiet while they are sitting in a pushchair is not getting the same [quality sedentary time].

"Children need to be given opportunities throughout the day to actively play and we should be reducing sedentary, passive screen time," she said.

Some TV programmes that encouraged young children to move about while viewing might be OK, she added, particularly if the parent or caregiver was also present to explain and join in.

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What do other experts think?

In the US, experts say children should not use screens before they are 18 months old.

In Canada, screen time for children younger than two is not recommended.

But UK guidelines set no such limit.

Dr Max Davie, from the RCPCH, said: "The restricted screen time limits suggested by the WHO do not seem proportionate to the potential harm.

"Our research has shown that currently there is not strong enough evidence to support the setting of screen time limits.

"It is difficult to see how a household with mixed-age children can shield a baby from any screen exposure at all, as is recommended.

"Overall, these WHO guidelines serve as useful benchmarks to help steer families towards active and healthy lifestyles - but without the right support in place, striving for the perfect could become the enemy of the good."

Dr Tim Smith, a brain development expert at Birbeck, University of London, said parents were being bombarded with conflicting advice, which could be confusing.

"There is currently no clear evidence for the specific duration limits proposed at this age range.

"While the report makes a potentially helpful step in distinguishing sedentary screen time from active screen-based games, where physical activity is required, this remains an oversimplification of the many ways young children and their families engage with screen media."

Image caption Paula Morton has two pre-school children

What can parents do?

Paula Morton, a teacher and mother of two young children, said her son learned a lot from watching programmes about dinosaurs and came out with "random facts about them".

"He doesn't just sit there and zone out," she said.

"He's obviously thinking and using his brain.

"I don't know how I would make the dinner, cook and clean if he didn't have something to watch."

According to the Royal College of Paediatrics and Child Health, parents can ask themselves:

Is screen time controlled?

Does screen use interfere with what your family want to do?

Does screen use interfere with sleep?

Are you able to control snacking during screen time?

If a family are satisfied with their answers to these questions, then they are likely to be handling screen time well, the college says.