Young people are increasingly likely to end up with sexually transmitted infections, experts say today as official figures are released showing record levels of STIs.

Those aged under 25 are most at risk because they are often vulnerable and lacking the confidence to negotiate relationships.

Data from the Health Protection Agency (HPA) indicates a worrying increase in sexually transmitted infections with sexual health clinics reporting 482,700 new cases in 2009, which is an increase of around 12,000 on the previous year.

While better and more widespread testing plays a part in the rise, there are concerns about young people. Two thirds of the STI cases were in females aged 15-24. These figures highlight the vulnerability of young women, said Dr Gwenda Hughes, head of the STI section of the HPA.

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

The peak age for a sexually transmitted infection is 19-20 for women and 20-23 for men. Alarmingly, significant numbers of young people are returning to clinics.

About 10% of the 15- to 24-year-olds treated for a sexually transmitted infection will be re-infected within a year.

"The numbers we're seeing in teenagers are of particular concern as this suggests they are repeatedly putting their own, as well as others', longterm health at risk from STIs," said Hughes.

Experts believe the figures show the need for more and better sex education for the young. Marie Stopes International, a sexual health NGO, said the figures were extremely concerning.

"It is clear that some young people are not fully aware of the prevalence of STIs and how they can protect themselves against getting one," said Helen Jenkins, contraception and sexual health specialist.

"We fear that STI rates may continue to rise, particularly among young people, without increasing access to comprehensive sex and relationships education in all British schools.

"Many teachers tell us that they don't have the confidence or information to teach sex and relationships education effectively, despite knowing that many of their students are sexually active."

Natika Halil from the FPA charity said the state of sexual health in the UK was complicated and a cause for serious concern. "The message from this data to the new government is that they mustn't be tempted to cut services and campaigns in sexual health, and ignore the urgent need for statutory sex and relationships education in schools," she said.

Sir Nick Partridge, chief executive of the Terrence Higgins Trust, called it "staggering" that infections had risen to almost half a million a year. "Until we improve sex education and give extra support to young people, they will continue to take avoidable risks with their sex lives," he said.

The HPA figures show clearly that people aged under 25 are most affected by STIs. Among women, 73% of new cases of gonorrhoea and 66% of new cases of genital warts involved under-25s.

Chlamydia was also most likely to be found in the under-25s (88% of cases), but that was partly to do with more sensitive tests and the targeting of the testing campaign to the age group.

But half of all new diagnoses in men also involved under-25s. That included 41% of male gonorrhoea diagnoses, 47% of male cases of genital warts and 69% of male cases of chlamydia. STI diagnoses were also high among men who have sex with men.

Genital warts and syphilis diagnoses have stabilised, the figures show, but gonorrhoea is on the increase, which raises a particular concern. Resistance to the main antibiotic used to treat the infection, cefixime, rose from 0.1% in 2005 to 10.6% in 2009.

Professor Cathy Ison, a gonorrhoea expert at HPA's centre for infections, said no new antibiotics were in the pipeline to treat the infection once cefixime "is no good". She estimated the antibiotic may be effective against gonorrhoea for only another five years, although that time could shorten if the bacteria develops new methods of resistance.

The HPA is investigating combination therapies to treat the bacteria and is encouraging pharmaceutical companies to develop new drugs.

"Potentially this means that in the case of gonorrhoea, practising safe sex may eventually be the only way of controlling the infection if new antibiotic treatments cannot be found," Ison said.

HPA advises that everyone should use a condom with any new sexual partner and visit a sexual health clinic after unsafe sex. People should also be screened for chlamydia every year and whenever they get a new partner, it says. Gay men should go for annual sexual health screening, including an HIV test.