"Porn has always been around but its format has evolved..."

As a European-wide project is launched to examine children's use of online pornography, figures show one in four teenagers with access to the net view porn at least once a month. For some it's an obsession, for others, an adolescent rite of passage.

Almost from day one, the internet became a byword for porn among the general public. And with it came concerns about the effect such ready availability of sexually explicit images would have on society, particularly on young people.

You don't have to go online to find sexual imagery, of course. These days, a passing glace at mainstream adverts, music videos and magazines will yield a good deal of bare flesh. But it's the graphic content of what can be found on the net - home to an estimated 250 million pages of pornography - and the ease with which it can be distributed, that concerns many parents.

Figures obtained by the BBC from Nielsen NetRatings reveal one in four of those aged 12 to 16 who goes online at home looks at sexual images at least once a month. That is one in three among boys and one in five girls.

A separate study by the London School of Economics claimed, this week, six in 10 children in the UK were regularly being exposed to porn, mostly as a result of viewing explicit websites accidentally. The study helped launch a Europe-wide research project looking into how young people use the net, called EU Kids Online.

It almost lodges itself into your mind, like a parasite sucking away the rest of your inner life Malcolm, 16

While some say porn is just a part of growing up for many teenagers, others believe it could be sowing the seeds of dysfunction.

Malcolm, aged 16, from the West Country, used to spend three to four hours a day watching pornography online, especially gothic-inspired sex, until he realised it was affecting his drama study and social life.

"It almost lodges itself into your mind, like a parasite sucking away the rest of your inner life and you kind of use it to answer everything and anything. It's a drug."

He sought help from a psychosexual therapist in London, Frances Emeleus, who discovered he was bullied at school.

"There was a crossed wire between Malcolm's sexuality and his anger, which is something I worked through with him in a straightforward and cognitive way," she says.

"The fact he was a teenager and he was self-aware helped me to find a route through to what was going on. He found a clean route to his anger as opposed to acting it out with a woman."

Following the treatment, Malcolm found the websites that he had been hooked on were suddenly less appealing.

Bluetooth sharing

But not everyone wants to kick the habit, or indeed thinks they have a problem. Darryl, 17, from Lincolnshire, views and shares porn with friends via Bluetooth on his mobile phone.

Darryl says looking at porn is normal

"If I didn't have work I'd be watching it constantly every day because it's something to do, like a drug," he says.

His female friends tell him they don't approve but he says it's normal.

"No-one can change my attitude to porn. I mean, I've been watching it for years. I'll carry on watching it, probably till I die. I see it as a normal thing and will always see it as a normal thing. No matter what people say."

Frances Emeleus says young people's relationships with porn are a new area of study and little is known about it. And while she believes porn can be a harmless and an integral part of growing up for hormonal teenagers, she thinks exposure to extreme forms is alarming.

"Using women or images as convenience in lieu of forming a relationship could impair the capacity to form a good relationship," she says. "I think particularly where the child doesn't witness a good relationship in the parental home, then the situation could compound itself."

If the porn is violent, then that behaviour could be enacted in real life, if social or moral feelings don't keep it in check.

"The more you encourage the objectification of women the more likely it is that sexual anomalies do emerge," she says.

But sex and relationship psychologist Petra Boynton says the porn "problem" among teenagers is often exaggerated and many young people are without their own internet access. And using extreme cases as examples does not help the debate.

It's expected from puberty that porn is a normal rite of passage for boys Dr Petra Boynton

For boys, exposure to porn can lead to anxiety about their bodies and their sexual performance, she says, while girls are "denied" access to it under the misconception they are not interested.

"It's expected from puberty that porn is a rite of passage for boys. They hit 12, get randy and look at boobs.

"For girls the introduction to puberty is to lecture them about getting pregnant. There's no expectation that they will be aroused - it's a passive sexuality that just isn't true." Her comments, though, conflict with Nielsen's findings that 20% of girls regularly access porn online.

The effect of pornography on society has long been a polarised debate, and the highly vocal feminist Catharine MacKinnon sounds a forceful voice against pornography, believing it dehumanises women.

Dr Boynton, however, calls for a wider critical debate on sexual education and young people.

Teens Hooked on Porn is on BBC Three on Thursday 8 February 2007 at 2100 GMT.