More than one in 10 children (12%) aged 12 to 13 have made or been part of a sexually explicit video and 9% are worried they are addicted to pornography, according to new research.
A total of 18% of children said they have seen porn images that have shocked or upset them, a survey of nearly 700 children for the NSPCC's ChildLine service found.
Dame Esther Rantzen, the founder of ChildLine, said that children as young as 11 had contacted the service with concerns about pornography.
She said: "Young people are turning to the internet to learn about sex and relationships.
"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.
"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."
Peter Liver, director of ChildLine, said children reported that watching porn made them feel depressed, gave them body image issues and put pressure on them to engage in sex acts they are not ready for.
A report by charity ChildWise in 2013/14 revealed that website Pornhub was among the top five favourite sites named by boys aged 11-16.
And young people post approximately 18,000 messages regarding exposure to porn on the ChildLine discussion forums every month.
Mr Liver said: "The Government recently proposed plans for children aged 11 upwards to be taught about rape and sexual consent as part of PSHE in schools.
"This would include discussion around what they have learnt from watching pornography.
"Across society, we need to remove the embarrassment and shame that exists around talking about porn - which is why we have launched a ChildLine campaign to help young people to make more informed choices."
ChildLine has created the Fight Against Porn Zombies (FAPZ) campaign to tackle the problem.
It will use animations to illustrate the implications of exposure to porn in both boys and girls.
Ruth Sutherland, chief executive of Relate relationship support charity, said: "We very much welcome ChildLine's new campaign, particularly because we know that children and young people do not feel that adults understand the online issues they face.
"In an IPPR poll last year, 61% of young people said adults are out of touch with young people's relationships and friendships, and 56% said adults find it hard to understand or help with online issues.
"That's why high quality, consistent relationships and sex education in schools is so important.
"We must get the right experts helping young people to understand what building blocks are needed for strong relationships, and ensure that what's being taught is applicable in the digital age. How easy access to porn is affecting young people - for example their sexual expectations and self-image - should be a vital component of this."