The toll that technology is taking on children's health was laid bare today in two major reports that linked screen time to 12 deadly cancers and short-sightedness.

A global review by the World Cancer Research Fund found rising exposure to smartphones, tablets and gaming consoles in childhood is driving long-term weight gain - identified as one of the key causes of cancer.

The new analysis looked at 80 studies involving more than 200,000 people in order to examine the causes of rising obesity and identified screen time in childhood as one of the chief culprits.

It follows findings from the WRCF linking being overweight or obese to 12 common types of cancer, including breast, prostate, colon, liver, ovarian, kidney and pancreatic disease.

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Elsewhere research by King's College London found that every hour a day playing computer games raises the risk of short-sightedness by three per cent, an effect dubbed 'digital myopia'.

In the last 50 years the number of children suffering from myopia has doubled from 7.2 per cent to 16.4 per cent, which experts say is fuelled by youngsters staying inside and staring at screens.

Previous studies suggest youngsters spend an average of eight hours a day using gadgets like smart phones, desktops and tablets and scientists warned they appeared to be having multiple effects on their health.

As well as being linked to lower levels of physical activity, it appears to dull satiety levels, leading to 'passive overconsumption' of snacks - often junk foods, which are often marketed via the same devices, the WRCF warned.

Their research identified children's screen time and consumption of sugary drinks as the two areas where the evidence to show increased risk of excess weight is strongest.

Caroline Cerny, from the Obesity Health Alliance said: "Eating too much unhealthy food and drink, with sedentary lifestyles contributes to overweight and obesity – this much is clear. We also know that when children spend time in front of screens, they are bombarded with junk food adverts."

She urged ministers to swiftly introduce a 9pm watershed on TV junk food marketing and similar restrictions online, a call that was echoed by cancer charities.

Sophia Lowes, from Cancer Research UK, said: "The report highlights the importance of acting early to help prevent cancer, by linking children's time in front of a screen to an increased risk of weight gain.

"Obese children are five times more likely to be obese as an adult, which is worrying because then they'll be at an increased risk of 13 different types of cancer.

"That's why it's vital we see a 9pm watershed on junk food adverts on TV and similar protection for children viewing adverts on-demand and online."

Chris Hammond from Kings College warned children were damaging both their eyesight and physical health by staying inside and using screens for close-up work.

"I think it is a significant concern," he said. "We are do believe that young children staying indoors and spending hours on screens is bad for eyesight not to mention obesity and fitness. "I think our concept in myopia is clearly that close work is a risk factor, and outdoor activity is protective."

In a linked editorial to the study, published in the British Journal of Ophthalmology, Dr Mohamed Dirani of the Singapore National Eye centre said short sightedness was a growing global problem.

In Asia up to 96 per cent of teenagers now have myopia. He called on new guidelines for face-to screen distances and for children to be taught lessons in body posture while using smart devices.

"The increased device screen time resulting from gaming, social media, and digital entertainment has led to a rise in sedentary behaviour, poor diet and a lack of outdoor activity," said Dr Dirani.

"The age of smart device uptake is getting younger, with many two-year-olds spending up to two hours a day on devices

"The use and misuse of smart devices, particularly in our paediatric populations, must be closely monitored to address the emerging phenomenon of digital myopia."