When he was 11, Max used to sneak into his mother’s bedroom when she was out and use her computer to Google words such as “cock and boobs”. What he discovered was a world of web-based pornography, which he admits gave him unrealistic expectations of sex and relationships.

“I was at that `in-betweeny’ stage between innocence and maturity,” says Max, now 18. “Watching porn gave me a grand idea of what sex is. I thought, `This is going to be brilliant’, that I would be sitting there eating a pizza and some girl would come in and have sex with me.

“Obviously, that’s completely unrealistic because the first time it’s a bit stiff and rookie and very brief. But you’ve got to learn about sex from somewhere and for me it was from friends and the internet.”

It is hard to know how many teenagers regularly watch internet porn, but the technology is moving so fast that many adults find it impossible to keep up with computer-savvy youngsters who can often get around parental controls and share pornography on their mobile phones.

Even hardcore and “special-interest” pornography is accessible. Damien, now 18, recalls how shocking it can be when seen for the first time. “Porn can spread around so easily. In Year 11, we sent videos around using our mobile phones and you’d look at it and wonder how anyone could do some of the things - the fact that young kids can get hands on that footage is worrying.”

Rebecca Avery, e-safety officer at Kent County Council, is concerned that youngsters are using pornography as a source of information and education. “It’s easy for their perception of what is normal to get warped if they have no comparison other than what they’ve seen on the internet or mates have loaded on to their mobile. We’ve got to teach them to be safe and discuss with them what they’ve seen.”

She has found that even primary pupils have accessed pornography, sometimes by using the history on their parents’ laptop or, as in the case of one nine-year-old, by working out the password to crack the parental controls on the home computer. At this age there can also be an element of bravado or cyber bullying.

“Sometimes they may get sent a link to something nasty or they’ll be dared by a mate to type `sex’ into the search engine,” she says. “If you type in anything to do with porn, it will come looking for you.”

As one of the first local authority e-safety officers in the UK, Ms Avery runs training courses for teachers and others working in schools, as well as providing education for parents and pupils.

She would like the issues of porn and social networking sites to be built into sex and relationships education as a matter of course, but she says some teachers are wary of such discussions, preferring to wait until individual pupils raise them.

She also finds that parents are often in denial, refusing to accept that their teenage child is accessing pornography or using their mobile phone to share sexually explicit images.

“No one likes to think of their child watching pornography,” she says. “The biggest problem with the internet is that children and teenagers don’t feel they can tell anyone if they see something they shouldn’t. Their assumption is the computer or mobile will be taken away from them. Parents often feel their children are safer on the computer because they’re at home - that may not be the case.”

That it may not be the case has prompted many schools in Kent to take up the e-safety training and support on offer, while pupils are encouraged to report anything online that makes them feel threatened or uncomfortable.

At one primary, this open-door policy led a 10-year-old girl to report an online friendship which immediately rang alarm bells with head Richard Sutton Smith. “They were asking to meet her,” he says. “It may have been innocent but we informed the police and told the girl to reply saying she wanted nothing more to do with the contact, who started sending emails with explicit content.

“Unfortunately, there were 11 or so other girls in the online friendship group who were also flooded with something like 20 links to explicit sites,” he says.

Mr Sutton Smith copied all the parents into the exchanges. “I knew that none of them would be aware that their child was gaining access to that sort of stuff simply by trying to have friends,” he adds.

When it comes to the Pandora’s box of internet porn, Ms Avery finds that older boys are more likely to push the boundaries, while girls tend to sign up on social-networking sites. Age restrictions seem to pose few barriers. Parents may give in to the pressure of the “all my friends are doing it” argument and sign up their children to sites such as Facebook even though they are under age - Facebook has a lower-age limit of 13. She even came across a four-year-old on Facebook, signed up by his 10-year-old brother, who in turn had been registered by his parents.

Many free pornography websites only require children and teenagers to tick a box to say they are over 18. Even where they have to complete a form to register, they can lie.

“We must make them realise that if they can lie, so can the other people they meet online,” says Ms Avery.

And much of this is hidden from both teachers and parents. “There’s loads of things adults don’t know about,” says Jemima, 14. “I’ve had friends who’ve met up with older guys they’ve met online and someone was asked to send pictures of herself topless - she didn’t because she didn’t want to embarrass herself, but she carried on talking to him.”

Another teenager tells how a 15-year-old girl in her school was videoed by her boyfriend having sex and the footage was Bluetoothed around his whole year. “She put a brave face on it although she was obviously very distressed. I don’t think her parents or teachers knew,” she says.

These are the kind of stories Vicky Gillings, of the Child Exploitation and Online Protection Centre (CEOP), hears daily. As a policing authority funded by the Government, it receives more than 6,000 reports of online abuse a year, some directly from children, others from parents, teachers or other professionals.

Every day, the centre has at least four cases on which it must take immediate action because a child is at such severe risk, either from a possible sex offender or because the situation has become so bad that they are threatening suicide.

“Children and teenagers are vulnerable online,” Ms Gillings says “They often don’t realise the risks of sharing sexually provocative images that can do the rounds, be manipulated and end up on a paedophile website.

“They are open to blackmail and coercion. They may be told that if they don’t do something, their account will be deleted or their profile page hacked into. The screen and keyboard give an illusion of safety and anonymity, but there are hugely powerful grooming techniques that can sexualise or normalise what this child is being asked to do.”

In Kent, it was an incident in which girls were asked to pose online for sexually explicit images that prompted the local authority to appoint an e-safety officer. The girls were targeted by a 21-year-old man in Canada, who has since been convicted and sentenced to three years’ imprisonment for extortion, blackmail and creating indecent images.

Ms Gillings believes there is huge pressure on young people to appear sexually available, not only from their peer group but from an increasingly sexualised celebrity culture. She says: “It only takes a celebrity to say they go on a site such as Chatroulette for youngsters to go on that site in the hope of meeting that celebrity.”

Ms Avery has come across youngsters organising Chatroulette parties. Intended to be a harmless random communication using webcam, the site was soon taken up by people masturbating or performing other sexual acts in front of the camera. She says that when youngsters stumble across such images, it difficult for them not to watch, even though it is real people doing real things in their front room.

“It’s what I call the `car crash effect’,” she says. ”`I can’t believe what I am watching’ - so they keep on watching.”

A lot of pornography holds similar fascination for sexually curious teenagers, even when it is accidentally accessed or unsolicited. One 14- year-old girl kept on being sent pop-ups and links to porn sites by another pupil during their MSN conversations. “It was horrible and you can’t get rid of those images,” says the girl, now 18.

While the assumption is that it is mainly boys who are consumers of pornography, girls are by no means aloof from it all, even if they see it as “a bit of a giggle”, something to add spice to a sleepover.

“I remember when I first saw porn at one of my friend’s sleepovers and we were just looking it up for a joke,” says one 17-year-old girl. “There was about 20 of us and literally you can just type in anything and see all kinds of porn. We were just messing around, but if you watch it when boys are about they often try to lad it up and it’s just embarrassing.”

She went on to watch more pornography with her gay best friend. “We were lying on his bed when he asked me if I wanted to watch gay porn - and I said yes,” she says. “I find it quite interesting, the different categories. Some can be sexy but some are ridiculous. I’ve been shocked by sadist pornography and I did see some men cut their girlfriends, but the more you watch it the less shocking it becomes - the human brain can only maintain the shock factor for a couple of seconds.”

Psychosexual therapist Dr Frances Emeleus, who has worked with youngsters addicted to pornography, is concerned that such desensitisation can normalise extreme sexual behaviour as well as leading to unrealistic expectations of themselves and their partner.

She is concerned that repeated exposure to sex at a young age can lead to “a split between feeling and excitement”.

“When there is no feeling, the pornographic image has to escalate to fill the whole sexual field and produce the same hit of excitement,” she says. “The danger of repeated exposure to pornography for young people is that they will see the other person as an exciting object to be discarded once they cease to be exciting. And when it comes to choosing a partner, you have a shopping list of what they should be like, what they should do or not do.”

But one 18-year-old, who is now in a steady relationship, believes his exposure to pornography has improved his sex life. “You want to experiment with things - different positions, different types of oral sex,” he says. “When you see bondage and that sort of stuff, you think `I’d never do it’ because it looks like it’s hurting, but when you’re having sex you may try different things like pulling your partner’s hair to see if it does turn you on, if it spices things up or not.”

Dr Emeleus is concerned that some girls are now being asked to do things during sex with which they are uncomfortable. “Sexual experimentation between young people has been going on for ever, but there’s something about the extreme images which is disturbing,” she says. “Learning about them is one thing, but ramping it up to avoid dealing with the issues of relationships and needing the buzz of that excitement is not a safe way to go.”

The proliferation of internet pornography may also be behind the rise in the number of girls as young as 13 coming to health professionals having had anal sex as a way of avoiding getting pregnant.

Simon Blake, chief executive of sexual health charity Brook, says anal sex is seen as less intimate than oral sex. “It’s a cultural problem that we don’t talk to young people about sex in ways in which they can protect themselves early on,” he says.

He believes pornography can create feelings of inadequacy around body image. Brook has also seen girls who are worried about having pubic hair after seeing waxed porn actresses. For boys, the feelings of inadequacy often revolve around penis size.

Mr Blake would like to see more age-appropriate sex and relationships education from primary school onwards. “We’ve got to start talking about the way the body works from early on so we can discuss the gritty issues later,” he says.

“We’ve got to help young people understand that pornography isn’t real life. It is a horrible way of boys learning about sex which objectifies women a lot of the time - and can be very frightening.”

• Sara Parker is the producer of `Sex, Porn and Teenagers’, a documentary to be broadcast on BBC Radio 4 on Monday, October 4 at 8pm.