Britain is in the grip of a sexual health crisis that is overwhelming the NHS and threatening the health of a generation of young people, public health specialists say.

New cases of sexually transmitted diseases (STDs) have soared in the past decade to more than one million a year, leading to growing waiting-lists and delays in treatment that are fuelling the epidemic.

The scale of the crisis is revealed by research showing sexual health clinics will need to triple their capacity to meet the Government's target to see and treat all patients within 48 hours by 2008. But specialists say the target will not be met because cash-strapped trusts are not ploughing enough money into sexual health.

The study, at a sexual health clinic in Leeds, found more than 600 appointments were needed to accommodate all those who sought treatment in one week but only 181 appointments were available. Almost 75 per cent of patients requesting a new appointment could not be seen, the researchers from Leeds General Infirmary say.

Doctors writing in the Journal of Sexual Health, which published the study, warn: "There is clearly a continued crisis in [sexual health] services in the UK. These are infectious conditions, and increased waiting will lead to increased transmission. Failure to deliver on this infection control measure will cost the public purse dearly in the long run."

Helen Ward, a specialist in infectious disease at Imperial College and joint author of the editorial in the Journal, said: "Primary care trusts are using the cash allocated for sexual health to pay their deficits. Up to half are definitely or maybe not going to spend the money on sexual health."

Since 1995, all STDs have soared, with a 62 per cent increase in new diagnoses to almost 700,000. Public health experts say the increase in HIV, gonorrhoea, syphilis and chlamydia is driven by increases in risky behaviour and a perception that the diseases are "trivial" and easily treated. Attitudes to sex have changed since the 1980s, when STDs fell in the wake of the HIV epidemic, as young people woke to the risks and changed their ways or used protection. But a new generation has grown up which has failed to learn those lessons and has a more casual attitude to sex.

The most common infection is chlamydia, which exceeded 100,000 cases for the first time in 2004. Experts say screening in some areas showed infection rates up to six times higher in the 16 to 25 age group than the 2 per cent of confirmed diagnoses for the age group in the general population, suggesting the true figure could be up to 600,000 cases. That would bring the total of sexual infections to more than one million.

Chlamydia mostly shows no symptoms but can cause miscarriage and infertility if undetected. A national screening programme is being phased in but covers only one in four NHS trusts, doctors say. New cases of HIV rose to 7,275 in 2004 and are projected to rise again 7,750 in 2005, the Health Protection Agency says. Cases of syphilis rose by more than a third to 2,234.

Ministers have pledged £315m to modernise sexual health services, including a £50m advertising campaign this year to warn young people of the risks.

Last November, the BBC Panorama programme found 93 per cent of sexual health clinics were unable to see and treat patients within 48 hours, and waiting lists of up to 28 days.