The new guidance, from all four chief medical officers of the UK, follows a review of the evidence about the impact of screen-time on children’s mental health and wellbeing.It found that heavy use of social media was associated with a doubling in depressive symptoms.The review said there was not enough published research to support any specific daily limit for screen time. But the guidance suggests parents should take a “precautionary approach” and not allow children to spend more than two hours at a time on smartphones and other gadgets.And social media providers will be told to introduce a raft of measures to tackle addictive and harmful behaviours, and prevent under-age use of sites, or face tougher laws. Dame Sally’s recommendations on screen use and social media: Leave phones outside the bedroom when it is bedtime. Talk about sharing photos and information online and how photos and words are sometimes manipulated. Parents and carers should never assume that children are happy for their photos to be shared. For everyone – when in doubt, don’t upload. Make sure you and your children are aware of, and abide by, their school’s policy on screen time. Keep moving! Everyone should take a break after a couple of hours sitting or lying down using a screen. It’s good to get up and move about a bit. Advise children to put their screens away while crossing the road or doing an activity that needs their full attention. Talk with your children about using screens and what they are watching. A change in behaviour can be a sign they are distressed – make sure they know they can always speak to you or another responsible adult if they feel uncomfortable with screen or social media use. Screen-free meal times are a good idea – you can enjoy face-to-face conversation, with adults giving their full attention to children. The country’s chief medical officer is backing a Duty of Care as she warns social media companies they must remove “addictive” technology or face new laws.Prof Dame Sally Davies spoke out as she issued the first official advice on screen time and social media – urging parents to ban smartphones from mealtimes and leave them outside of bedrooms at night.The new guidance suggests parents need to take more control over the digital habits of their sons and daughters and find out what content they are watching.And it suggests using tracking features to monitor how much time children spend looking at screens, or using social media.It comes as Health Secretary Matt Hancock warned social media providers to “clean up your act” – or face new laws to force them to comply with a duty of care.“We are masters of our own fate as a nation and we can and must legislate to make sure this amazing technology is used for good if social media companies won’t work with us,” he warns, writing in the Telegraph.Today he will meet senior figures from Instagram, Facebook and Google and urge them to take action to remove images, videos and messages that glamorise suicide or self-harm. Dame Sally said: “Technology is an unavoidable aspect of modern life and technology companies have a duty of care. They must make more effort to keep their users safe from harm, particularly children and young people.”The Telegraph has been campaigning for a statutory duty of care to protect children from online harms, which is now under consideration. The chief medical officer said parents should talk to their children about the content of what they are watching, and look out for changes in behaviour. As well as the guidance for parents, she has asked the technology industry to introduce changes to establish a “duty of care” to young users, or face legislation. She also recommends that parents try using tracking features which measure how much time they and – with permission – their children spend looking at screens, or on social media,And families are urged to have “screen-free mealtimes” with “adults giving their full attention to children” away from televisions, tablets and smartphones.The guidance, published by the chief medical officers, also says parents should never assume a child is happy to have their photo published online and should talk to them about the fact that photographs can be manipulated.The review found 38.1 per cent of teenage girls spending more than five hours a day on social media suffered from depressive symptoms, compared with rates of 18.1 per cent among those spending between one to three hours on such sites. Levels of depression among teenage boys rose from 6.8 per cent to 14.5 per cent, in the group spending longer on such sites.The review could not prove a causal link.But Dame Sally urged parents to take steps to control their children’s use of social media, while further research is carried out. These include changes in the way sites are structured to “remove addictive capabilities” which encourage users to get hooked.This could mean getting rid of functions like “auto-play” which mean content is continuously delivered, or pushed with nudges that “you might like” particular videos or games, and praise for logging in regularly.Other measures include effective age verification methods, age-appropriate advertisments, and the removal of harmful content such as bullying or images of self-harming.And the technology industry will be asked to contribute to funding on research on the impact of screen time, and to share anonymised data to improve the scientific evidence base about the potential risks and benefits of social media.Shirley Cramer, chief executive of the Royal Society for Public Health said the advice would help parents who were “crying out for help to protect their children and help them navigate the Wild West of the digital world.”Dr Bernadka Dubicka, from the Royal College of Psychiatrists said: “The chief medical officer is right to be cautious about how long children are spending looking at screens rather than talking to their friends or getting a good night’s sleep.“We do not yet have enough evidence to draw a definite causal link between amount of screen time use and mental health problems but it is clear that some of the content that young people are viewing online, such as pro-anorexia, suicide or self-harming content, can be incredibly harmful.” Want the best of The Telegraph direct to your email and WhatsApp? 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