Young adults under the age of 25 are responsible for the majority of nearly half a million new cases of sexually transmitted infections in the UK in 2009.
A total of 482,696 cases were reported in sexual health clinics last year, which was increase of 12,000 from 2008.
The Health Protection Agency, which collected the figures, said young women are “particularly vulnerable” and frequently “lack the skills and confidence to negotiate safer sex”.
Under-25s accounted for two-thirds of new STI cases in women, including 73% of new cases of gonorrhoea in women and 66% of new cases of genital warts.
A huge 88% of new cases of chlamydia - the most common STI in the UK - in women were discovered in the same age group.
There was a 7% increase in chlamydia cases among both sexes across all age groups between 2008 and 2009, while cases of gonorrhoea were up 6%.
Genital herpes cases increases by 5% while genital warts cases stabilised with a 0.3% drop. Syphilis cases fell by 1%.
While more chlamydia testing in the community - and more sensitive tests for other STIs - accounted for much of the rise, the HPA said it was clear many people were still having unsafe sex.
Young adults and gay men are the groups most worrying the HPA.
Re-infection is also an issue, with at least 11% of 16 to 19-year-old women in England becoming re-infected with an STI within one year of being treated for a previous one.
The same is true for young men, with 12% of 16 to 19-year-olds becoming re-infected within one year.
Head of the HPA’s STI section Dr Gwenda Hughes said: “These figures also highlight the vulnerability of young women. Many studies have shown that young adults are more likely to have unsafe sex and often they lack the skills and confidence to negotiate safer sex.”
Marie Stopes International’s contraception and sexual health specialist Helen Jenkins said: “We are extremely concerned about the reported rise in the rate of sexually transmitted infections (STIs) in the UK. 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.
“We fear that STI rates may continue to rise, particularly among young people, without increased 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.”