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There were almost half a million new cases of sexually transmitted infections (STIs) in the UK last year, figures show.

Experts at the Health Protection Agency (HPA) say young people are most affected.

And one in 10 of 15-24 year olds with an STI become infected again within a year.

Health ministers said they would look at what more could be done to increase young people's awareness of risks.

The 482,696 new cases represent a 3% rise from the 2008 figures, continuing a "steady upward trend" that the HPA said had been seen over the past decade.

Urban and deprived areas have the highest rates of STIs. Hotspots include Hackney and Lambeth in London, as well as Nottingham, Manchester and Blackpool.

The rise is in part due to more testing, and the use of tests which are more sensitive at picking up signs of an infection - but experts believe unsafe sexual behaviour is also part of the story.

Dr Gwenda Hughes, an STI expert at the HPA, said: "These figures highlight the vulnerability of young women.

"Many studies have shown that young adults are more likely to have unsafe sex. Often they lack the skills and confidence to negotiate safer sex.

"Re-infection is also a worrying issue. Teenagers are repeatedly putting their own and others' long-term health at risk."

There were 217,570 diagnoses of chlamydia in 2009 - a 7% increase on the previous year. Cases of genital herpes went up by 5% to 30,126.

And diagnoses of gonorrhoea have gone up by 6% from 16,451 cases in 2008 to 17,385 last year.

Dr Colm O'Mahony, a consultant physician in sexual health, told ³ÉÈË¿ìÊÖ Radio 5 live, that the safe sex message was still not getting through to young people.

"In general, most STIs occur in young people because they lack the knowledge and self-esteem to actually avoid getting sexually transmitted infections - and that's what we've been shouting about for years," he said.

"We really need proper sex and relationship education in schools, and it needs to be a statutory obligation or this relentless increase will just continue unabated."

The HPA says the bacteria which cause gonorrhoea are becoming more resistant to the most commonly used antibiotics.

Professor Cathy Ison, from the agency's Centre for Infections, said: "We could see gonorrhoea becoming a very difficult infection to treat within the next five years.

"The infecting bacteria are very versatile. We're trying to encourage companies to develop effective new antibiotics."

Sexual health charities described the figures as "alarming" and joined the HPA in urging people to use condoms, and to have a health check if they have had unsafe sex.

Natika Halil, from the FPA, said: "Young men don't wear condoms and it appears it's young women who end up with the infection.

"The message from this data to the new government is that they mustn't be tempted to cut services and campaigns in sexual health, or ignore the urgent need for statutory sex and relationships education in schools."

Health Minister Paul Burstow said: "Left untreated, STIs can lead to infertility.

"We're going to look at what more can be done to increase young people's awareness of risks, to prevent infection and to access screening and treatment."