Nearly half a million new cases of sexually-transmitted infections (STIs) were diagnosed in the UK in 2009, according to Health Protection Agency (HPA) figures.
A total of 482,696 cases were reported in sexual health clinics last year, up almost 12,000 on 2008.
The HPA says that young women are "particularly vulnerable" and often "lack the skills and confidence to negotiate safer sex".
Two-thirds of new STI cases in women hit those aged under 25, including 73% of new cases of gonorrhoea in women and 66% of new cases of genital warts.
Meanwhile 88% of new cases of chlamydia - the most frequently occurring STI in the UK - in women were discovered in the same age group.
The number of chlamydia cases among both sexes rose by 7% across all age groups between 2008 and 2009, while there was a 6% hike in gonorrhoea figures.
Genital herpes cases were up by 5% while genital warts 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 concerning 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.
Helen Jenkins, a contraception and sexual health specialist at Marie Stopes International, said: "We fear that STI rates may continue to rise 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 relationship education effectively, despite knowing that many of their students are sexually active."
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