A tenth of 12 to 13-year-olds fear they are "addicted" to pornography, an NSPCC ChildLine survey has concluded.
One in five of nearly 700 youngsters surveyed said they had seen pornographic images that had shocked or upset them, researchers found.
The charity also says that 12% of those surveyed said they had taken part in, or had made, a sexually explicit video.
It says that viewing porn is "a part of everyday life" for many of the children who contact its helpline.
ChildLine has launched a campaign to raise awareness and provide advice to young people about the harmful implications of an over exposure to porn following the survey results.
One boy under the age of 15 told ChildLine that he was "always watching porn, and some of it is quite aggressive".
He said: "I didn't think it was affecting me at first but I've started to view girls a bit differently recently and it's making me worried.
"I would like to get married in the future but I'm scared it might never happen if I carry on thinking about girls the way I do."
A girl, who is now 17, told the BBC that she was sexually assaulted by her boyfriend when they were both 12 years old.
"He thought it was OK on some level," she said.
"I felt dirty, confused, shocked.
"Pornography isn't just a 10-minute video - it has consequences."
The ChildLine Fight Against Porn Zombies (FAPZ) campaign uses a series of animations looking at the implications of overexposure to porn for boys and girls.
The animations link to a range of information and advice to help young people understand the effects of replicating pornographic content in real life and to protect them from putting themselves at risk.
Peter Liver, director of ChildLine, said that it was important to talk openly about the issue.
"Children of all ages today have easy access to a wide range of pornography," he said. "If we as a society shy away from talking about this issue, we are failing the thousands of young people it is affecting.
"We know from the young people who contact ChildLine that viewing porn is a part of everyday life, and our poll shows that one in five 12 to 13-year-olds thinks that watching porn is normal behaviour.
"They tell ChildLine that watching porn is making them feel depressed, giving them body image issues, and making them feel pressured to engage in sexual acts they're not ready for."
He welcomed the announcement last week of plans to teach children from the age of 11 about rape and sexual consent as part of personal, social and health education (PSHE) in schools.
"Our campaign clearly complements this proposal," he said.
"Across society, we need to remove the embarrassment and shame that exists around talking about porn - which is why we are launching this activity and helping young people to make more informed choices."
'Damaging and upsetting'
Dame Esther Rantzen, ChildLine's founder, said it was shocking that children as young as 11 are approaching the helpline with concerns about pornography.
"Young people are turning to the internet to learn about sex and relationships," she said.
"We know they are frequently stumbling across porn, often unintentionally, and they are telling us very clearly that this is having a damaging and upsetting effect on them.
"Girls in particular have said they feel like they have to look and behave like porn stars to be liked by boys."
Dame Esther said that improved education was vital.
"We absolutely have to talk to young people about sex, love, respect and consent as soon as we feel they are ready, to ensure that they gain a proper perspective between real-life relationships and the fantasy world of porn," she said.