A few months before their wedding, David Prior (not his real name) told his fiancée Sue his biggest secret. Although the couple had a good sex life, and were committed to a future together, he was addicted to visiting prostitutes.

Perhaps surprisingly, the wedding went ahead. "I was horribly shocked, but I thought that with some therapy he'd get over it," says Sue, 42. Fifteen years and two children on, the couple are still together – but David's sex addiction, too, is with him still. "I don't think we could possibly have imagined that it would be as long-term, or as difficult, as it has been," says Sue.

David and Sue are clients of a sexual psychotherapist, Paula Hall, who last month published the UK's first comprehensive guide to what sex addiction is – she defines it as "a pattern of out-of-control sexual behaviour that causes problems in someone's life" – and how its sufferers can be helped. No one knows how many sex addicts there are in Britain but, says Ms Hall, in her professional judgement it's hugely on the increase.

"I run training sessions on understanding sex addiction, and four years ago I might have been training six therapists a year," she says. "Now I'm doing 20 sessions annually, with an average of 30 therapists at each. And when you ask them why they're here they all give the same answer: more and more clients are presenting with sex addiction, and they want to know more about how to help them."

No one understands what the rise in sex addiction is entirely about but internet porn, Ms Hall says, has got to be part of it. "Porn is like the gateway drug. Just as with cannabis and cocaine, many people will use the gateway drug and never become addicted; but others most definitely will. And unlike drugs or alcohol, users don't even realise they're dealing with something that might prove to be addictive."

The easy availability of porn gives people in high-stress jobs a way of dealing with their pressurised lifestyle. "We're seeing more soldiers who've done tours in Afghanistan and have used internet porn as a means of escape," Ms Hall says. "And as with alcohol, using porn to soothe the pressures of life isn't bad in itself – unless and until it becomes addictive, and is the only way you can escape from your problems."

In researching her book, Ms Hall surveyed 350 people who described themselves as addicted to sex, 25 per cent of whom were women. "The proportion of women addicts surprised me," she says. "And what surprised me even more was the number who are using porn: 90 per cent of the men I surveyed, and 74 per cent of the women, said they were heavy porn-users. We have this idea that women are into relationship sex while men are more visually stimulated, so this seemed to fly in the face of that."

The biggest problem for sex addicts, Ms Hall says, is that it's seen as a moral deficiency rather than a mental illness. Also, despite the increase in people seeking therapy, most of those affected still try to deal with it alone – and the fallout can be devastating. "In my survey I asked people what the worst consequence of their addiction was, and the answers were truly terrible," she says. "People had lost their families, they'd lost their homes and gone bankrupt, they were depressed, even suicidal, and they felt unable to embark on a proper relationship."

One respondent said he was a 30-year-old virgin who has never had a girlfriend or dated. "Porn has distorted my view of real women and I now think my natural libido is not what it should be," he explained. "Porn has been a comfort blanket for my anxieties but at the same time helped to increase them while stopping me from facing up to my problems and living my life to the full."

What's also interesting about her research, Ms Hall says, is how young sex addiction starts: of the people she surveyed, 40 per cent were under 16 and nearly 10 per cent were under 10 years old when their problems started.

For David Prior, though, the problem started later: he was in his mid-twenties when he started to visit prostitutes and realised his problems were deeper than just getting a sexual kick. "I was in a very unhappy relationship, and I now realise that was echoing the difficult relationship I'd had with my mother," he says. "Seeing a prostitute became a way of escape; and even when I met Sue and started what became a good relationship with her, I still needed the fix of seeing sex workers.

"The strange thing was, and is, that it's never been about fun or enjoyment: the thing I craved, and still do crave, is the feeling of shame. One of the biggest ironies about sex addiction is that it's only marginally about sex; like all addicts, what it's really about is being unable to process or deal with something difficult in your life, whether it's unhappiness, boredom or frustration."

For sex addicts, the nature of their addiction gives them additional problems. "If I was an alcoholic or a drug addict, people would be sympathetic and would want to help," David says. "But with sex addiction it's completely different; people moralise it, they can't possibly deal with it. I couldn't possibly tell anyone I know about my addiction, so it's a very lonely thing to deal with – and of course as for all addicts, the first step to overcoming it is admitting it's going on.

"Another problem is that, unlike other addictions, you can't decide to give it up: I can't say I'll be celibate, because I'm a married man and I want a healthy sex life with my wife."

For now, David says, his addiction is under control. It's at least two years since he visited a sex worker, and these days he limits his "fix" to seeking out sexual thrills, and then walking away from them. "I act it out by going to some dodgy place – but when I'm offered a hand job or whatever, I leave without going through with it," he says. "I get the sense of shame, but I don't actually participate in the sex act." But still there are dangers. "I'm a senior manager, and I have a responsible job," he says. "If anyone who knew me at work saw me coming out of one of those places, that could be the end of everything, the end of being able to provide for my family, the end of my career. The stakes are very high."

So how can sex addicts be helped? A big breakthrough for many, says Ms Hall – who chairs the Association for the Treatment of Sex Addiction and Compulsivity – is realising they're not alone. "Many people with this problem believe they're the only one with it – so finding out that there are good people out there struggling in exactly the same way really boosts their self-esteem," she says.

Therapy focuses on understanding what addicts are anaesthetising themselves against, and finding healthy, alternative ways to meet their needs. Beyond this, Ms Hall says, the key lies in finding practical ways of preventing them from relapsing. "I advise clients to block adult content on the internet, and to avoid people or places that trigger their need for sex. But the big problem is that it's a lot harder to avoid these triggers in our age than in the past, because porn is accessible in so many ways, and in so many situations."

'Understanding and Treating Sex Addiction' by Paula Hall is published by Routledge (£19.99)

Are you at risk of sex addiction?

Paula Hall identifies six factors that make people more vulnerable to sex addiction:

* Early sexualisation: it's known that early alcohol use changes the chemistry of the brain in ways that make alcoholism more likely, and it's possible that something similar could happen to a brain that is exposed to sexual images and behaviour before it is ready to react to them.

* Adolescent isolation: experiencing trauma between the ages of nine and 13, or feeling isolated as a teenager for any reason, can increase the chances of becoming addicted to sex.

* Over-controlling parenting: people who have been too controlled as children are less able to handle risk-taking, and this could predispose them to sex addiction.

* Limited modelling of emotional regulation: growing up without good role-models for emotional regulation can leave people dependent on external factors to manage their emotions, because they can't handle them for themselves.

* Childhood shame: thinking of sex as shameful leads people to think of sex as secretive, and makes it harder to normalise it as a healthy part of ordinary life.

* Family secrets: a hidden issue in a family can set a person up for a situation where having two separate parts to life is normal, even attractive.