A fifth of children in Britain have been bullied, with cyber bullying now more common than face to face taunts, a major new study has revealed.
There has also been a sharp spike in the number of youngsters who are exposed to hate messages, self-harm and pro-anorexia websites, according to the research.
Experts warned the report, Net Children Go Mobile, painted a worrying picture of the dangers young people face online, and called for teachers, doctors and parents to step up their vigilance.
Sonia Livingstone, one of the report's authors, said smartphones and tablets meant children are on the internet 24/7, leaving them unable to escape bullies.
She said: "Bullying is happening both ways. It is happening at school and when they go home they are on Facebook and targeted. It is really hard to escape. That is the new phenomenon. Kids are on their Facebook 24/7.
"There is research that shows that if cyber bullying is anonymous then it is easier to lash out at others. And on the internet, people might think they are just having a bit of a laugh, but you can't see how the person on the other side is really feeling and responding.
"The internet is so fast, so bullying messages and images that are put out there can spread so quickly it is hard to contain them and it can escalate."
The study is produced by the EU Kids Online research programme at LSE university and looks at the internet habits of children across 25 EU countries.
It showed that in 2013, 21% of children in Britain aged nine to 16 had been bullied, with more girls than boys reporting being picked on.
For the first time, more youths said they were cyber bullied (12%) than picked on face to face (9%), with social networking sites such as Facebook being the most common forum for online taunts.
This is a significant shift from 2010 - when the last study was conducted - when 16% of children reported being bullied face to face, 8% on the internet and 5% on their mobile phones.
And m ore impressionable children are surfing the internet to find toxic websites that promote self harm or anorexia.
Nearly a third of 11 to 16 year-olds (29%) have seen potentially negative user generated content, with 23% reporting seeing hate messages online, while 17% have seen self harm sites.
This is a sharp increase from 2010, when just a fifth (19%) of children said they had seen negative content, with 13% reporting that they had seen hate messages, while 8% said they had seen pro-anorexia sites and 6% has seen self-harm sites.
Prof Livingstone branded this sharp rise "shocking" and warned these sites "normalised" self harm and eating disorders, and risked encouraging teenagers into hurting themselves.
She said girls beginning secondary school were particularly vulnerable to the damaging sites.
She said: "The pressure is on to be a certain type of person, to look a certain way, or think sexy looks a certain way. For some it takes them down a pro anorexia route and some find a thrill in self harming, and that is really upsetting.
"The internet gives this a visibility and almost a normalcy. It might be that one teenager in ten self harms, but if you go on a site and see a lot of people talking about it, it reinforces your perception about how common it is."
She said the internet "can be part of the solution" and called on teachers and parents to better educate young people to the dangers of the web.
Shaun Kelly, head of safeguarding at Action for Children, said: "Young people's increasing exposure to negative user-generated content, such as bullying via social media or sites about self-harming, is a sobering reminder of the potential risks to children online.
"Increased smartphone and tablet usage means parents and teachers cannot always supervise young people's activity, however talking to young people about internet use can effectively build resilience and protect against potentially damaging or disturbing content.
"Adults must show an interest in all aspects of young people's online activities, not just when they are concerned."