Surgeon Bill Ledger, clad in faded green scrubs, whispers a prayer in Latin and prepares to operate. Before him, a young woman lies anaesthetised. "Stop pulling on the cervix, please," he instructs one of his six-strong team of assistants.

Ledger carefully inserts a tiny camera lens into an opening below the woman's belly button. Above them, a television monitor flickers into life and an ovary, dangling white in the womb, appears on the screen.

Sian, 24, may be infertile as a result of a sexually transmitted infection (STI) that she contracted two and a half years ago. The operation will establish whether the infection, chlamydia, has permanently blocked the tubes between her ovaries and her womb.

"This spiderweb-like stuff shouldn't be there," says Ledger, tracing a series of white, glutinous strands around the ovary. "Those are adhesions, sheets of scar tissue - inflammation from chlamydia."

A member of his team takes a large, plastic syringe containing blue dye, inserts it into a catheter that leads into Sian's uterus, and begins to apply pressure. All eyes remain fixed on the TV screen, searching for traces of blue to emerge from her Fallopian tubes and flow into the womb. But there's nothing. They push harder on the syringe. Again, not a hint of blue on the monitor. Her tubes are completely blocked.

Two months ago, fertility specialist Lord Winston said the evidence that chlamydia caused infertility was "dubious". I challenge Ledger on this, asking him whether - as a leading fertility expert himself - he can be sure it is chlamydia that has blocked Sian's tubes.

"Doctors are never 100 per cent sure about anything," he says, "but as sure as you can be - given the findings and the history - she has tubal damage secondary to chlamydia. I don't think you can get those adhesions in any other scenario, except from a chlamydia infection." Ledger declares the operation over and calls for the next patient. With incisions repaired, Sian is wheeled out of the operating theatre.

No one knows exactly how many women are undergoing these operations across the UK, such is the paucity of data. But Ledger says that his hospital processes at least 100 such cases every year.

"There are 30,000 IVF treatments a year in Britain now, and chlamydia is responsible for a quarter or a third," says Ledger. "There are probably 5,000 or 10,000 cases of people needing IVF treatment because they have had chlamydia problems in the past."

"I'm not denying the extent of the problem with chlamydia and its consequences," she says, "but we have a strategy in place [the Government's new screening programme, aimed at getting all sexually active 16- to 24-year-olds tested for chlamydia] to tackle the issue head-on."

No area of public health in the UK has suffered a more dramatic decline in recent years than sexual health. In the past decade, annual rates of gonorrhoea have more than doubled to over 20,000; new cases of HIV have spiralled to 7,000 a year, almost treble the rate 10 years ago; and syphilis, the great scourge of European armies of old, has shot up 1,497 per cent.

But it is chlamydia that is causing the greatest alarm, with a 222 per cent increase in diagnoses in the past 10 years. It is estimated that nearly half a million sexually active young adults in the UK carry the infection, many without even knowing it, as the symptoms, such as cystitis and mild lower abdominal pain, are non-specific. Chlamydia shows no symptoms in about 70 per cent of infected women and 50 per cent of infected men. But left untreated, it can trigger major health problems, including ectopic pregnancies, pelvic inflammatory disease (PID) and - as in Sian's case - infertility. Chlamydia can also lead to arthritis, mostly in men, and research is being carried out to establish whether it can affect male fertility. Babies born to infected mothers are at risk of developing conjunctivitis or pneumonia soon after birth.

However, the genitourinary (GU) clinics that tackle these armies of disease, as Parliament's Health Select committee noted in 2003, are "one of the poorest-resourced, most stretched and least well-staffed areas of the NHS". When Panorama journalists called each of the 269 clinics in the country, posing as patients seeking both routine and urgent appointments, they discovered that the STI epidemic has left the clinics in disarray. "We are full to almost overspilling," explains one receptionist in the documentary, as she tries, in vain, to arrange a quick appointment.

"Unfortunately, the first appointment I have is almost six weeks away. I know it's shocking," apologises another. Some clinics simply refuse: "We're not taking any new patients at the moment. We just haven't got the doctors in the clinic."

With record waiting times for appointments, treatments are delayed and infections are being passed on (research suggests that 30 per cent of those with symptoms continue to have sex).

In many respects, we have only ourselves to blame. This crisis stems, in part, from an increasingly sexualised culture in which people are opting to take more sexual partners and more sexual risks. But policy-makers know that they, too, must share the blame. After coming to power, even though infections were on the increase, Labour removed sexual health from the list of key national health priorities. It then took them four years to publish a national sexual health strategy, but it lacked sufficient funding, according to some senior GU clinicians.

Lately, the Government has recognised that it must give sexual health a greater priority. It has allocated £300 million extra funding, in the wake of its Public Health White Paper, to help rebuild the service and to support the national chlamydia screening programme. It has also set a target of allocating every patient an appointment within 48 hours by 2008 and, in the next few months, a major sexual health advertising campaign will be launched. Above all, the need to educate a new generation about the dangers of unsafe sex is imperative. Sian had never heard of chlamydia before she was diagnosed with it.

Five hours after her operation, Prof Ledger goes to her hospital bed and gently wakes her. As she opens her eyes and looks up, he begins his familiar monologue: "You had your laparoscopy," he says, "and the womb and ovaries are pretty normal. But you've got adhesions in your pelvis from chlamydia. We tried hard to get dye to go through your tubes but we couldn't. So I think the reason you're not getting pregnant is because your tubes aren't working and the way around that is to put you through the IVF programme."

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