Even as a consultant with 15 years' experience in the field of sexual health, Chris Wood sometimes finds it hard to believe his eyes. "You look at people who come into the clinic and you think: 'Blimey, how the hell can they be walking around like that?' People have come in with horrible discharge around their genitals and they won't have told anyone about it. Get this: I was doing a sexual health assessment on one patient. I asked the guy, whose underpants were sticking to him, to tell me when he last had sex. 'Yesterday,' he said. I just couldn't believe it. How could he still go out and have sex with these sort of symptoms?"

Wood works out of two London hospital clinics and is on the frontline of the fight to tackle a rapid surge in sexually transmitted infections (STIs) in the UK. He is just one of a weary band of health professionals who are trying to get to grips with a 116% rise in STI's in the past decade. It is, Wood says, "an epidemic. We are facing a full-scale public health crisis, and nowhere near enough is being done about it."

Wood says nothing much shocks him anymore when it comes to STIs - even when people present with clear symptoms but have left it quite late to be seen. In any given week, he says, he diagnoses a whole range of infections from HIV, to genital herpes, to chlamydia. What does surprise - and anger - him, he says, is that "tackling sexual disease in a developed nation like Britain is in such a state of crisis".

He points to the evidence. STIs are the fastest growing health condition in the UK, with treatment costing an estimated £1bn a year. Between 1992 and 2002, new diagnoses for chlamydia jumped by 195%, gonorrhoea was up by 148%, and syphilis by a staggering 380%. Meanwhile, the number of cases of HIV diagnosed over the same period doubled. By the end of 2002, the Health Protection Agency (HPA) estimated that just under 50,000 people were living with HIV in the UK, many of them unaware of their condition.

To a degree, the crisis stems from a culture of risky sexual behaviour. The vast majority of cases of STIs are, unsurprisingly perhaps, attributable to unprotected sex. According to medical experts, including the HPA, sexual health messages about the link between using condoms and preventing infection have failed to get through - especially to some younger people, who are the most sexually active.

"People may have had a drunken one-night stand and not used a condom," Wood explains. "Or it might be that they are in a steady relationship, but their partner has been carrying an STI contracted from a previous partner. I really believe that, in the population as a whole, there is a remarkably low level of knowledge about STIs, and we encounter it every day in clinics."

Ignorance about how STIs spread, and about how to identify telltale symptoms, exacerbate the problem, Wood says. In the case of chlamydia, for example, the symptoms can be completely undetectable without a test. "People can carry it for a long time and be passing it on to other people without even knowing they have it," he points out. "But it can be serious. Chlamydia can lead to extreme pain and to reduced fertility."

Like many health professionals, Wood contends that the curiously British attitude to sex - that it is either shameful or a bit of a joke - fosters ignorance and undermines efforts to tackle rises in STIs. For a start, he says, the attitude reinforces the stigma around sexual infection, meaning that people are often afraid or unwilling to talk openly about it. "People feel guilty and embarrassed, and some may leave it quite late before seeking help."

Whatever the prevailing culture, it requires more than an increase in careless sexual encounters and promiscuity to generate a crisis of the sort currently facing the country. Wood says the government needs to "radically rethink" sex education in schools and to put more resources into prevention. But there is a more immediate and pressing problem, he says: a chronic underinvestment in the clinics that diagnose and treat people with STIs.

There is a growing consensus among sexual health experts that genito urinary medicine clinics (GUMs) are so starved of resources and funding that they are buckling under the strain of increased demand for their services. Despite a much-trumpeted National Strategy for Sexual Health, introduced by the government in 2001, according to the British Association for Sexual Health and HIV (BASHH) Britain's sexual health crisis has spiralled out of control because the capacity for treatment is woefully inadequate.

This is the reality: more people than ever are being diagnosed, and queues at GUM clinics are unmanageable as a result (workload at GUM clinics shot up by 55% between 2002 and 2003). Opening times in Britain's 256 GUM clinics are sometimes limited to 21 hours a week. People are often turned away. Many clinics don't even have suitable buildings: some operate out of Portakabins.

And the picture gets grimmer. Waiting times for appointments at GUM clinics tend to be measured in weeks, rather than days. It is not uncommon for patients to wait as long as eight weeks to be seen. "People can find their symptoms take a turn for the worse in days, never mind weeks," Wood says. "And, yes, there is the chance that those left undiagnosed will still be having sex and spreading an infection."

It isn't easy to anticipate which disease will spread where. There are marked regional differences: for example, experts are at odds to understand why Hull has far more cases of drug-resistant gonorrhoea than anywhere else.

Against this rather depressing backdrop - and in anticipation of the government's white paper on health, published yesterday - the Terrence Higgins Trust (THT) spoke to Wood and other sexual health professionals, as well as patients, to gauge the reality of pressures on the frontline. The document that resulted - a compilation of their stories, called Tales from the Clinic - paints a shocking and distressing picture. "For years, the pressure had been building," reads Wood's entry. "Queues were getting bigger, starting earlier, snaking around the building. More and more people were being turned away ... there were longer waiting times inside the clinic. There was more frustration, shorter tempers, shouting and abuse from angry patients. More stress, more staff sickness, more strain all round, burn-out and madness."

Anurse who began working in sexual health at the dawn of the Aids crisis in 1985 describes similar experiences. In her contribution she says: "Over the past few years, clinics have become increasingly busy and, as a result, it is very difficult to get a same-day appointment. Twenty years on, I am still working in a Portakabin."

The fallout from the colossal strains on the system are not restricted to staff. For patients confronted with stressed and under-resourced workers, it can mean not only long waits for appointments but also distressing, impersonal consultations without time for much-needed counselling and advice. "The general service in my clinic is deteriorating, and individual attention at the clinic is not good enough because of the large number of patients accessing the services," says one long-term HIV-positive patient.

Another patient, who tried to access services for the first time recently to allay fears of infection, says her experience was "the most frightening" of her life. She encountered staff too frenetic to properly explain anything to her, a two-week wait for results, and no counselling.

Sexual health charities such as THT have lobbied hard for urgent government action - and cash. In this week's health white paper, the government has set its sights on maximum waiting times of 48 hours. A good sign, campaigners say, but as Paul Ward, deputy director of THT, points out: "Forty-eight hours is better than we have now, but in 1989 average waiting times were 24 hours, so what does that tell us about government commitment?"

The government has been warned by its own experts, too. In its first report, published this summer, the Independent Advisory Group on Sexual Health and HIV concluded: "To call the increase in STIs an epidemic is not unfounded ... GUM services would soon be overloaded without further support and investment."

Underlying much of the crisis is that where money has been made available, it is not necessarily reaching the clinics that need it. BASHH claims that roughly a third of the £8m allocated in 2003-04 to Primary Care Trusts (PCTs) with lead responsibilities for GUM clinics never reached the clinics, but was siphoned off to pay for what the trusts thought were more pressing priorities. Sexual health is simply not high enough up the agendas of either government or PCTs, according to BASHH president Angela Robinson.

Both BASHH and THT advocate a complete restructuring of funding to make sure that more money is made available. Contrary to current government thinking, they argue that funds should be ring-fenced and go directly to the clinics. Robinson says the structure of PCTs regionally makes the system "a postcode lottery", with the level of treatment a patient can expect dependent upon where they live. "It is just not acceptable," she says. "We need real government commitment, not lip service."

In his consulting room in north London, Wood sums up the feelings of many in the sector. "The system is a real mess, and I think it will continue to get worse," he warns. "Something needs to done about it - and fast."

Ten things you ought to know about sexual diseases

1. HIV is the fastest growing serious health condition in the UK and is at an all-time high. There are an estimated 50,000 people in the country living with HIV. This could rise to 100,000 by 2008.

2. One in 10 women is believed to be infected with chlamydia. Younger women are more susceptible to some STIs than older women.

3. Outbreaks of individual diseases vary dramatically across the county. For example, Manchester and London have seen the highest incidences of syphilis. It can be difficult for sexual health workers to predict in what areas a disease will take hold.

4. Young people are susceptible to STIs and HIV because they have more sexual partners. Ethnic minorities suffer disproportionately because of poor access to sexual health services.

5. Failure to diagnose and treat STIs early can lead to serious long-term health problems. An estimated 31% of people with HIV are undiagnosed.

6. Each year, 1.5 million new STI and HIV cases are seen at clinics. In 1997, the government removed sexual health and HIV from its list of national priorities.

7. HIV rates in injecting drug users attending clinics remains low, at below 1%, but the prevalence of the hepatitis C antibody stands at 14% for those who began injecting between1999 and 2002.

8. Most of the newly diagnosed cases of HIV are among heterosexuals. Experts believe three-quarters of the cases originated in Africa.

9. The workload at genito-urinary medicine clinics has gone up by 70% since 1997, but funding has gone up by only one-third. Some 20% of clinics are based in Portakabins. Waiting times can be as long as eight weeks.

10. Around 30% of people waiting to be seen at GUM clinics continue to have sex, despite being symptomatic.

· The Terrence Higgins Trust is at www.tht.org.uk The Health Protection Agency is at www.hpa.org.uk