Alarming numbers of Lambeth’s young people are having unprotected sex, a health watchdog has warned, as shocking figures show the borough has the second-highest rate of sexually transmitted infections(STIs) in the UK.

Thousands of residents, especially youths, are contracting infections each year, the Health Protection Agency (HPA) said, leading to fears of a health timebomb as spending on sex education looks set to be axed.

Lambeth is already battling one of the highest levels of teenage pregnancy and HIV infection in Europe, and experts warned young people were risking contracting HIV thanks to a relaxed attitude to protection.

HPA estimates one in 20 people in Lambeth aged 15 to 24 have chlamydia, and an NHS Lambeth spokesman warned the same young people were “at a real risk” of catching HIV through having unprotected sex.

The agency also said in total 2.3 per cent of Lambeth residents had an STI, meaning the rate among the sexually active population was significantly higher.

Dr Helen Maguire, an STI expert at the HPA, warned of a trend of poor sexual health and said the statistics showed Lambeth’s youths were taking part in unsafe sexual activity “due to a lack of skills and confidence to stop risky sexual behaviour”.

She said improved education and access to sexual health services, including screening, was vital in deprived inner-city areas like Lambeth, where STIs were most prevalent.

Her comments came as health pressure group London Health Emergency (LHE) predicted funding for sexual education and prevention would be slashed, amid cuts to the borough’s health budget.

LHE chairman Geoff Martin said: “This would have a disastrous effect on NHS budgets long-term, as infection levels would rise and more would need to be spent on treatment.”

NHS Lambeth promised fighting STI infections, along with HIV/Aids and teenage pregnancy, would remain the trust’s top priorities, but could not guarantee funding would not be cut because of budget pressures.

A spokesman said the new data would be analysed to target services toward “at risk groups”, but the organisation had already recognised high level of STIs in Lambeth – and had a five-year strategy to improve sexual health services.

He said an ongoing reduction in teenage pregnancies and a successful chlamydia screening programme, which in 2009-10 achieved the highest uptake nationally for screening 15 to 24-year-olds, suggested the strategy was delivering, and likely to have contributed to the high rates, as more people were being tested.