Kids should then take “exercise breaks”. The move comes amid growing alarm at a generation hooked on social media.
Campaigners are likely to demand the crackdown – to be unveiled by Dame Sally Davies next Thursday – goes even further.
Research earlier this week found under-fives spend four hours and 16 minutes a day glued to screens – including online, ,gamingand TV.
Seven in ten of those aged 12 to 15 took smartphones to bed.
And a fifth of children aged 8-12 are on social media – despite supposed bans on under-13s. The new guidelines revealed by James Forsyth in today’s Sun follow an official request from Health Secretary Matt Hancock
The Tory high-flyer last weekend demanded social media giants remove suicide and self-harm material from their sites after the father of a 14-year old teenager blamed Instagram for her death.
Ian Russell said Molly – who committed suicide two years ago – had been looking at social media content about depression, self-harm and suicide.
In a blistering letter to web firms, Mr Hancock said: “It is time for internet and social media providers to step up and purge this content once and for all.”
He added: “Let me be clear, we will introduce new legislation where needed.”
Ofcom earlier this week claimed so many children were glued to YouTube that they had given up other activities such as drawing or going out on their scooters.
Some youngsters said they went out less – because of it was “too much effort” when they could just hook up online.
Researchers warned while many kids go online to watch harmless videos they end up watching unsuitable content by accident.
The guidelines will pile pressure on web giants to introduce a cut-off for under-18s
Education Secretary Damian Hinds last November urged parents to lead by example and drag themselves away from smartphones and tablets.
US academics last summer said children learn smartphone habits from their parents – and it was important adults learn to “unplug”.
The Chief Medical Officer’s view is guidance rather than regulation. Sources compared it to official advice such as “eat your five a day” on vegetables.
The advice will say that kids shouldn’t spend no more than two hours at a time on social media – and that they should do exercise before going back on again.
But insiders said the political argument will be whether the social media companies self-regulate – such as logging out under 18s once they’ve been on for two hours.