After 15 years in what she thought was a happy, loving relationship, Jessica was horrified to discover that her partner had become obsessed with online pornography.

"We had always had a very good sexual relationship and never had a problem, so it was the last thing that I would have suspected," said the 37-year-old businesswoman, who has asked for her surname to be withheld.

"I was so shocked and we had huge rows about it so many times, but he used to try to justify it by blaming me for not giving him enough attention after our daughter was born."

The couple eventually separated last June after her husband had an affair with a woman he met online. But Jessica, who lives in East Anglia, blames online pornography.

It took her a while to realise why their phone bills were so high. "He had always been into computers, and when we were on dial-up we'd get these massive phone bills - and at the time I couldn't understand why," says Jessica, who has a five-year-old daughter.

"Later on it all made sense, and with broadband the world was his oyster. I would walk into the room and see my husband staring at the computer screen and would often catch a glimpse of an image. I began to notice the different sites he had been visiting and it got to the stage where he would be downloading stuff at all hours of the day and night."

Jessica is one of an increasing number of women who are finding that their partners are enjoying secret sex lives through online pornography. New figures obtained exclusively by The Independent on Sunday reveal that record numbers of British men are becoming hooked on internet pornography, with more than nine million adults using sex websites last year.

But it is not just men who are downloading graphic images to satisfy their sexual appetites and surfing the net for sex aids. There has been a rise of a third over the past year in the number of women visiting X-rated websites every month, an increase from 1.05 million to 1.38 million.

These figures, based on data from world-leading internet analysts Nielsen NetRatings, highlight in accurate detail for the first time the full scale of the nation's obsession with porn. Until now, the extent of the market has not been properly documented, because it is a notoriously secretive industry.

Despite the "no sex, please" reputation of the British, therapists say that the sexual appetite of men and women in this country is just as great as that of the French or Spanish. And using the web to access sexually-stimulating images is an increasingly popular way of satisfying this appetite. The images are often found on sites generated in the US.

Thaddeus Birchard, a psychotherapist, says the internet encourages addiction to porn because it is cheap to download and more anonymous than buying a magazine from a shop or visiting a sex club.

"The addictive power is related to its affordability - it's cheap, and also it is anonymous and accessible," said Dr Birchard.

"You can use it in your bedroom; you don't have to walk outside your house to find porn. The internet is also infinite - it's like falling into a vortex. There is a tendency for users to go for hit after hit and eventually get totally caught up in it."

Research shows that men use picture-based websites, and women use erotic chatrooms because men's desire tends to be visual, whereas women are more relational. However, women are more likely to view use of porn as an act of betrayal and many experts believe that people who use pornography are unable to sustain proper relationships with real women.

Counsellors working for Relate report that use of internet porn is a cause of tension and conflict for as many as 40 per cent of the couples who come to them seeking help with sexual problems.

Christine Lacey, a senior counsellor for the relationship guidance charity, said that normal women feel under pressure to compete with the "perfect" women who are portrayed in pornographic images.

"[Internet porn] is often a symptom of other problems in the relationship, but it is also an issue in itself," she said.

"On the internet there are no bounds. You can look at the most perfect bodies doing the most unusual sexual practices and having sex whenever you want to watch. What normal woman can compete against that?

"I think the effects of porn are ultimately the same, but they are exacerbated by the internet because it can be accessed entirely in private. If people are watching this sort of thing all the time without contact with other people, it can normalise it."

Pat Jones blames online pornography for the collapse of the relationship with his girlfriend. The 47-year-old sports coach says that his obsession with internet sex sites even drove him to the verge of suicide.

Three years ago, he suffered a complete breakdown and now attends weekly meetings organised by Sex and Love Addicts Anonymous.

"The internet is where my existing obsession with porn really took off," says Mr Jones, who lives in the north of England. Before then I had made do with occasional videos that people would bring in to work. I would be online for up to 10 hours at a time and wanted more and more. I saw just about everything except for child and snuff stuff."

He likens his obsession to alcoholism. He says: "It is like drink- you build up your tolerance to it. It was getting worse and I was looking for harder and harder stuff. When my girlfriend was out working, I'd be sitting on the computer, but by this time I didn't care about the consequences. She left me, and at around that time my father died and I lost my job. I collapsed and hit rock bottom. At that point I was suicidal and wanted to die."

Even now, he still finds it hard to suppress his desires and believes that society is more sexualised than ever before.

"Sex addiction is rife in our culture. What is now top- shelf stuff would have been hardcore porn 20 years ago. Society is far more sexualised in an addictive manner than it used to be," says Mr Jones.

"I think there are more sex addicts out there than there are drugs and drink addicts, and I think that is slowly becoming more recognised. People need to know that there is a way out of the hell-hole of sex and love addiction. There are support groups out there now."

The explosive growth of pornography on the internet has led to Britain's porn barons getting on board. Paul Raymond, who publishes Mayfair, Men Only and Club International, has a strong presence with his website sexclub.co.uk. The UK has also been overwhelmed by the liberalisation of the pornographic video and DVD market.

The internet is threatening to kill off the traditional, soft-porn magazine industry which formed in the liberal, permissive atmosphere of the late Sixties and spread into the nation's newsagents.

The men who once dominated the British porn magazine industry have now sold off their top-shelf titles - sometimes in a bid for respectability.

There is no academic research to show that there is a direct link between the use of porn and violent sexual crime. But there are concerns that the more time people spend searching for porn on the internet and viewing it, the more likely they are to inadvertently become exposed to illegal material such as graphic images of child sex abuse.

The Internet Watch Foundation, which combats online child abuse, warns that the more people get hooked on web porn, the further they go into websites and the more they are directed to hard-core material which can eventually open the doors to child pornography.

A spokesman said: "Sadly, these images - instances of children being abused - on the internet facilitate a multi- million-pound business, and in order to sell more images, they advertise their services - spamming members of the public who want nothing to do with such material."

Oliver James, the clinical psychologist and author, said that pornography interferes with people's ability to sustain a real relationship.

He said: "Men who look at images of models find that afterwards they love their partner less, and feel less satisfied with their lot. If a man looks at a lot of pornography - in which women are essentially slaves - it interferes with their ability to sustain a real relationship.

"Pornography provides an escape for people who find it difficult to cope with intimacy, but the more they use it, the less they can relate to other people."

Janice Lamb, a sexual and relational psychotherapist, says that women often see the porn as a "third person" who has come between the partners in a relationship.

"A person who has used a lot of pornography might feel that they can't relate to real women because they don't behave like fantasies," said Ms Lamb, who is a member of the British Association for Sexual and Relationship Therapy.

"Part of the attraction of pornography is that you're not dependent on another person. But if you become dependent on it, you are missing out on the richness of a mutual experience."

But Tracey Cox, the TV presenter and relationships expert, said that rather than ruining relationships, porn saves "a hell of a lot of them", and that most normal people enjoy looking at sex books and enjoying sex fantasies. "Intelligent adults know that porn isn't real sex - it's naughty, titillating fun, and it can stimulate the senses when they get jaded," she said.

"In a relationship we can get desensitised to each other. Porn can give you kinkiness and titillation, without having to actually try swinging or a threesome."

Additional reporting by Anthony Barnes, Jonathan Owen and Martin Hodgson

Film & DVD: Hard-core films flood market

More hard-core pornographic films are now being passed by censors each week than used to be passed in a year. A flood of porn DVDs has now hit the market, with almost 6,500 hard-core movies available to be bought legally.

Until a legal ruling in 2000, only 25 soft-core films a year were given certificates. But last week alone, 28 films were passed, and they contained far more explicit and prolonged penetrative sex scenes.

With 282 licensed sex shops selling the films, classified R18, for upwards of £20, the industry is valued at about £200m a year.

Liberalisation has also led to a homegrown industry developing, using hard-core stars such as April Long, directors such as Ben Dover and distributors such as Relish. Production values vary from amateur compilation tapes to glossy films with a hint of a plot and dialogue. Most DVDs sold in Britain come from the biggest market in the world, the US. The world's top-selling titles are from the US and include Pamela Anderson and Tommy Lee's home movie, Debbie Does Dallas and Flashpoint, with the porn starlet Jenna Jameson.

However, the UK industry is under siege from the internet and DVD piracy. Anna Kieran, of porn distributor Hot Rod Productions and a senior figure in the Adult Industry Trade Association, said: "Business is down by about 30 per cent compared to last year. On every street corner there are people selling counterfeit DVDs."

A legal decision last year had a major impact on the UK porn industry, stopping sales of R18 films by inland mail order. It is still possible to buy from Europe, so larger British firms have set up warehouses offshore. Smaller ones have to sell through sex shops.

Anthony Barnes

Magazines: Top-shelf titles under pressure

Britain's once-thriving soft-core porn magazine industry is shrivelling away, killed off by the worldwide web's unparalleled flood of pornography into Britain's homes.

After the liberalisation of the sex industry in the late 1960s, "porn barons" such as Paul Raymond and David Sullivan built empires with top-shelf titles such as Men Only and Mayfair. Scores of often poorly made magazines were once on sale, but now sources in the publicity-shy industry, which refuses to publish any circulation figures, admit that Britain's porn magazine industry is in freefall. In five years, monthly sales have halved to around 500,000 copies.

The internet is the major cause of this collapse. Britain's small porn industry failed to keep step with modern technology, and found itself quickly overtaken by mainly North American rivals. Smaller British porn magazines have simply stopped appearing, leaving only the biggest names. One porn publisher admitted: "I don't think that there's that many titles being published right now."

David and Ralph Gold, the millionaire brothers who own the Ann Summers chain of sex shops and a share of Birmingham City FC, are said to have sold about 60 titles. David Sullivan has also sold off titles, as has Richard Desmond, who moved out of magazines to concentrate on more buttoned- up businesses. Smart move. The company publishing his old titles, Remnant Media, is losing money.

The industry source argued that Britain's porn magazines were hampered by Britain's relatively strict censorship rules and the strictures of Britain's dominant magazine distributors, WH Smith and John Menzies, which refuse to circulate more extreme US titles.

Severin Carrell

The Director: 'Women want to buy porn. I make films for them'

I used to be very anti-porn, because if you were a feminist in the Eighties that was the way you saw it. But I was walking down Old Compton Street in Soho and I realised my anger was jealousy: I was jealous that men had their sexuality catered for and women didn't.

There are a lot of women who want to buy porn, but they are put off because it is too much from a male perspective. I make films from a female point of view. I make the characters more three-dimensional, get them to act naturally. We have a four- or five-minute scenario at the beginning: they are not just porn stars, they are characters who you see having sex. There is more foreplay, we have eye contact between the characters, the camera angles are from a female point of view and the men are good-looking. You can tell the women are having a good time. It doesn't look as if they are just doing it for the money.

I don't make stuff to make women look bad. I'm not saying porn doesn't have a negative side - of course it does, but this gives us a possible route forward. When women start buying it, they become more choosy and the quality goes up.

I think there is less stigma attached to the porn industry nowadays. The rise of Ann Summers, the popularity of the Chippendales and the Margi Clarke sex show - they were all precursors to its acceptance by women. They now buy and choose and have become an economic force."

Anna Span, 33 is the UK's leading female porn director. She made her first sex film while studying at Central Saint Martins, London