In 2006, a total of 376,508 new STIs were diagnosed - up 2.2% on 2005, the Health Protection Agency found.
Young people aged 16-24 made up the bulk of cases of some of the most common STIs, including chlamydia.
The HPA also warned of a continuing HIV and STI epidemic in gay men.
Sexual infection diagnoses have been almost continually rising since the 1990s.
Experts called for more action and funding to tackle the issue.
According to the latest figures, the total number of sexually transmitted infections rose to more than 620,000 - up 2.4% on 2005. This figure includes recurrent infections.
An estimated 73,000 adults are now living with HIV in the UK. A third of the people in Britain with HIV, don't know they have the virus.
HIV transmission seems to be a particular problem among gay men - the HPA anticipates that there will have been just over 2,700 new diagnoses of HIV infection among this population in 2006.
The sexual health of young adults in general has worsened, with increases in sexually transmitted herpes and warts viruses in 2006.
One in 10 young adults screened for the sexually transmitted infection chlamydia in that year tested positive for it.
Young adults are disproportionately affected by STIs, accounting for 65% of all chlamydia, 55% of all genital warts and 48% of gonorrhoea diagnoses in genitourinary medicine clinics across the UK in 2006, the HPA found.
Professor Pete Borriello, director of the HPA's Centre for Infections, said: "We have to get the message across that a casual shag should not mean syphilis, gonorrhoea, chlamydia or any other STI.
"We need to change attitudes towards condom use. It should be clunk clip every trip."
He recommend that gay men should have regular HIV tests, STI clinic attendees should be tested for HIV at every visit and young sexually active adults should be screened for chlamydia annually and after a partner change.
Simon Blake, chief executive of the sexual health provider for young people Brook said: "This is a strong signal, yet again, that we must do more to improve the sexual health of our young people."
Nick Partridge, chief executive of HIV and sexual health charity Terrence Higgins Trust said: "Despite our best efforts and those of hard-pressed clinics, the HIV epidemic in the UK is getting worse. It's clear that we can't tackle it on our own.
"We need more investment in HIV prevention, more HIV testing in local communities and stronger national leadership. This is a real test for national government and local health services - and one we can't afford to fail."