The majority of children are exposed to pornography by their early teens, leaving many at risk of becoming "desensitised" to its impact, a study has found.
Around 53% of 11 to 16-year-olds have encountered lewd material online, nearly all of whom (94%) have seen it by the age of 14, according to research by Middlesex University.
More than a quarter of that group (28%) had first been confronted with the explicit content when they were only 11 or 12.
Smartphones were a common way for children to access X-rated sites as 33% of those surveyed first saw pornography on a handheld device.
The survey, commissioned by the NSPCC and the Children's Commissioner for England, also discovered it was more likely for youngsters to find material accidentally (28%) than seek it out (19%).
The child protection charity said an entire generation of children was at risk of being "stripped of their childhoods" through exposure to pornography at a young age.
More than three-quarters of children surveyed - 87% of boys and 77% of girls - felt pornography failed to help them understand consent, while half of boys (53%) saw it as a realistic depiction of sex.
One 13-year-old girl told researchers: "A few of my friends have used it for guidance about sex and are getting the wrong image of relationships."
Another said: "It can make a boy not look for love, just look for sex, and it can pressure us girls to act and look and behave in a certain way before we might be ready for it."
Some children's approach to sex was also informed by pornographic scenes, with more than a third (39%) of 13 to 14-year-olds boys saying they wanted to copy the behaviour they had seen.
One of the 13-year-old boys said: "One of my friends has started treating women like he sees on the videos – not major – just a slap here or there."
Anne Longfield, Children's Commissioner for England, said: "In recent years the Government and internet providers have done much to restrict children's access to pornography but there is no room for complacency – parents, teachers, regulators and the digital industry must not let up in efforts to do so.
"They must also do more to help children who do see porn to understand what they have seen. We know from the research that very many children are shocked, confused or disgusted by what they see and it is our duty to help them to question, challenge and make sense of it."
Peter Wanless, NSPCC chief executive, said: "A generation of children are in danger of being stripped of their childhoods at a young age by stumbling across extreme and violent porn online. Frighteningly, some children are growing up believing that they should emulate the behaviour they see in porn, which can have a damaging effect on their relationships.
"Industry and Government need to take more responsibility to ensure that young people are protected. Some companies have taken the initiative when it comes to online safety, we will continue to put pressure on those that have not yet done so."
Researchers talked to more than 1,000 children aged 11 to 16 as part of the study, which is the most extensive look at the impact of porn on secondary school pupils in the UK to date.
A Department for Culture, Media and Sport spokeswoman said: "Keeping children safe online is one of Government's key priorities. Just as we do offline, we want to make sure children are prevented from accessing pornographic content online which should only be viewed by adults.
"In the forthcoming Digital Economy Bill we will bring in legislation that will require companies providing pornographic material online to make sure they have a robust age-verification system in place, so that those accessing their websites are over 18."