Diagnoses of sexually transmitted infections rose to almost half a million in England last year, with the highest rates in those aged under 25.
According to Public Health England, there were 448,422 diagnoses in 2012 - a rise of 5% from 2011.
The data shows too many people are putting themselves at risk through unsafe sex, it says.
People aged under 25 made up 64% of all chlamydia and 54% of genital warts diagnoses in heterosexuals in 2012.
New diagnoses of gonorrhoea rose 21%, which is a concern given the growing global threat of antibiotic resistance.
More sexually transmitted infections (STIs) were being diagnosed and treated than ever before, with improvements in screening particularly for gonorrhoea and chlamydia among young adults and men who have sex with men, said Public Health England.
"However, these data show too many people are continuing to have unsafe sex, putting themselves at risk of STIs and the serious consequences associated with infection, including infertility," said Dr Gwenda Hughes, head of STI surveillance.
"Ongoing investment in programmes to increase sexual health awareness, condom use and testing, particularly for groups at most risk, is vital."
Increases in STI diagnoses were seen in men who have sex with men, including a 37% increase in gonorrhoea diagnoses.
Although partly due to increased testing in this section of the population, ongoing high levels of unsafe sexual behaviour probably contributed to this rise, said Public Health England.
Among the advice it offers to help reduce the risk of STis is always using a condom when having sex with new partners, reducing the number of sexual partners and avoiding overlapping sexual relationships, an getting tested regularly if in one of the highest risk groups.
Lisa Power, policy director at the Terrence Higgins Trust, said: "Britain's sexual health is on a slippery slope and without sustained local investment across the whole country it can only get worse.
"With nearly half a million new STI diagnoses last year, it is vital that local authorities invest in ensuring that STI prevention and testing services are readily available at a local level."
Dr Janet Wilson, president of the British Association of Sexual Health and HIV, said the ongoing rise in diagnoses among men who have sex with men and high levels of gonorrhoea transmission were of particular concern, especially as data pointed to the emergence of antibiotic resistant strains of gonorrhoea.
"It is therefore critical that easy and open access to specialist sexual health services are sustained in all parts of the country, as well as improved prevention work," she said.