No one wants to talk about it, but a dramatic rise in cases of the sexually transmitted disease chlamydia - up by 250 per cent in the past decade - could be making a generation of young women infertile

Cases of chlamydia have risen by almost 250 per cent over the past 10 years, prompting fears of a sexual health crisis among young people.

Official figures to be published this week will show that more than 105,000 people were diagnosed last year with the sexually transmitted disease (STD), compared with 32,288 cases in 1995.

The startling rise in cases of the infection, which can lead to infertility and ectopic pregnancies, has alarmed health professionals, who blame increased casual and unprotected sex among young people. It will disappoint health ministers, who have made tackling STDs a priority.

The figures, to be published by the Health Protection Agency, will increase fears of a lurking "silent time bomb" of cases, as the disease usually shows no symptoms.

Chlamydia, which is transmitted through unprotected sex, is most prevalent in young people aged 15 to 24. It can be treated easily by a course of antibiotics, but around 70 per cent of women who have the infection do not realise they are carrying it. Infertility caused by the untreated infection can be devastating, and costs the NHS more than £100m a year.

Pauline Beddoes, a senior nurse at Marie Stopes, said the organisation's clinics have seen a sharp rise in detection of the disease over the past year. "In females, about 80 per cent have no symptoms and most people only act when they have symptoms," she said. "It is common between 15- to 24-year-old females, and males between 18 and 26. It's sleeping with more partners and the lack of condoms that has led to the increase."

The Government launched a national chlamydia screening programme in 2003, and has already tested more than 80,000 under-25s. Over 6,000 free chlamydia screening kits have also been handed out at Boots in a government-funded pilot to make testing more accessible to 16- to 24-year-olds. One in 10 of those screened proved to be positive for the disease, and ministers favour the tests being made more easily available.

The Tories yesterday blamed the Government for presiding over an "epidemic" of STDs while cutting the budget for sexual health awareness campaigns. Andrew Lansley, the party's health spokesman, said the Government had spent £53m on sexual health advertising since coming to power, compared with £139m in real terms spent during the last nine years of the Tory government.

"Epidemic levels of sexually transmitted diseases and a rising rate of HIV diagnoses show the human consequence of the Government's failure to maintain sexual health information and services for a new generation of young people," he said. "In the last year, NHS financial pressures hit the Department of Health's own budget for sexual health campaigns."

Experts believe the increase in STDS means clinics will be unable to meet the demand for treatment. The Government has set a target to treat all patients within 48 hours by 2008. But pressure on NHS finances means sexual health budgets often do not receive enough investment.

There are around five million sexually active people aged between 14 and 24 in the UK. Cases of STDs among the general population, including chlamydia and gonorrhoea, have soared in the past decade to more than a million a year.

The infections have been associated with an increased risk of HIV transmission and infection. And women with chlamydia are at risk of developing pelvic inflammatory disease, which can lead to infertility.

Infected pregnant women who are not treated can pass chlamydia to babies, causing eye inflammation or pneumonia. Men can suffer pain and inflammation of the testicles.

Dr Gillian Vanhegan, senior medical officer at the sexual health advisory clinic Brook, said chlamydia was "very easy to treat... [But] the possible long-term side effects of infertility and ectopic pregnancy and pain are quite serious."

Milly's story: 'It hurt so much I had to go to hospital'

It was Christmas Eve five years ago when Milly Davis discovered she had contracted chlamydia. Like thousands of women in the UK, the 26-year-old administrator had lived with the disease for years, unaware she was infected. But unlike most men and women, who have no symptoms, Ms Davis was not so lucky. She found herself in terrible pain, and a visit to her doctor revealed that she had pelvic inflammatory disease - which can lead to infertility.

"It turned into a horrible painful thing around my stomach. I had to go to hospital it was so painful," she recalls.

It was only after her doctor recommended a chlamydia test that it emerged the sexually transmitted disease was the cause of the pelvic inflammation.

"I wouldn't have known unless I had had the test. I could have had it for years and years," she said.

Ms Davis, then 21, was prescribed antibiotics to combat the infection. At around the same time, she discovered she was not the only one to have fallen victim to the "silent STD".

"My boyfriend was in hospital a few weeks before. His testicles had swollen up massively. He got tested... and found out he had chlamydia," she said.

"When I was in hospital there was a girl who was told she was infertile because she had had chlamydia for years and years. She did get pregnant later but found it was ectopic and she had to have a hysterectomy. It was a real horror story."