Children as young as 11 are becoming addicted to obscene material on the internet, according to Welsh campaigners.
The problem, they say, is so bad it has led to some instances of children simulating extremely violent or graphically sexual scenes at schools.
Concerns are growing after figures revealed three quarters of children have unaccompanied access to the internet and 800,000 can view adult content, 14 % of whom are younger than 10.
It comes as the government considers tightening controls on internet pornography.
The Prime Minister is to discuss plans where home broadband customers can choose to opt out of having access to sites with adult content.
Research suggests as many as one in three under-10s has seen pornography on the web and Welsh teaching bosses say it is a serious child protection issue.
Four in every five children aged 14 to 16 admit regularly accessing explicit images and video footage on their home computers.
The situation is alarming to many parents and campaigners warn that exposure to these images inflicts “serious mental harm” on minors and prevents them from forming healthy adult relationships in later life.
Mid and West Wales AM Rebecca Evans said access to pornography has a profound and negative effect on our children.
“This is one of the most serious and pressing public health issues affecting children and young people,” said Ms Evans.
“A cross-party parliamentary report showed children and young people are accessing online pornography and websites showing extreme violence.
“It found that it is having a serious impact on the children and young people’s development, from diminishing their sympathy for victims of sexual assault to reducing their inhibitions and making them more vulnerable to abuse or exploitation.
“We already successfully regulate British TV channels, cinema screens, High Street hoardings and newsagent shelves to stop children seeing inappropriate images – and mobile phone companies are able to restrict access to adult material – so why should the internet be any different?”
Meanwhile, school leaders have voiced fears that the rising use of social networking may be providing quick access to inappropriate online material for children on sites many parents assume are ‘safe’.
Anna Brychan, director of NAHT Cymru – which has gathered for its annual conference this weekend – said new technologies and social networking sites present schools, parents and pupils with a huge range of challenges.
“One of the difficulties is that very young children are often considerably more technologically savvy than their parents,” she said.
“The capacity of even very small children to enter a dubious and damaging virtual world can easily be underestimated by parents and carers. School leaders are telling us that this manifests itself in children simulating extremely violent or graphically sexual scenes at school.
“Influences on children now extend far beyond their families and peer group. Schools have become very responsive to the challenges they face in terms of dealing with the hugely more complex lives of their pupils.”
A spokesman for NAHT said: “Internet safety is taught in schools; controls are in place and there is oversight of pupils’ use of technology. It is a fast-moving world and schools know they’ll have to remain vigilant and one step ahead of their pupils.”
But the NAHT said that, for parents, control can be much harder.
“We think that increasing parental awareness of these issues and helping them to make sure their children benefit from the positive aspects of technology but are not damaged by its negative aspects needs some serious consideration,” the spokesman said.
Only 3% of pornographic websites currently require proof-of-age before granting access to sexually explicit material, and two-thirds do not even include any adult-content warnings.
Keith Towler, Children’s Commissioner for Wales, said there is no quick fix solution to safeguarding children and young people online .
“It is impossible for us to ignore the fact that children and young people are using the internet to play, socialise and to learn in every part of their lives,” he said.
“We can make a difference by investing in teaching children and young people to become confident citizens of this new digital space. We must embrace new technologies, learn about them and use this knowledge to help our children and young people develop the skills to become safe and responsible internet users.”