The number of sexually transmitted infections in England, Wales and Northern Ireland rose by 4% last year, figures show.
The Health Protection Agency said cases of chlamydia - the most common sexually transmitted infection - jumped by 9%.
Overall, 708,083 people were diagnosed with an STI in 2003.
It is thought complacency about condom use, increased numbers of sexual partners and long waits for treatment may all be helping to fuel the trend.
If people have a number of partners, if they have casual sex and don't take precautions, they are putting themselves and others at risk
Professor Pat Troop
However, the rate of increase is starting to slow down. The number of new cases of gonorrhoea fell by 3%.
And experts believe that at least some of the rise in overall cases can be attributed to greater public awareness, and more people coming forward for testing.
The number of STIs increased throughout the 1990s, with chlamydia seeing a 140% rise in just six years. Last year 89,818 people were diagnosed with the disease.
Chlamydia can cause infertility. However, it can be difficult to notice, as often no symptoms are apparent.
The HPA found big variations in STI diagnoses across the country. For example, there were outbreaks of syphilis in Manchester and London.
Professor Pat Troop, HPA chief executive, said the idea that teenagers were most at risk was wrong - the highest rates of infection were often among people in their twenties.
She said: "If people have a number of partners, if they have casual sex and don't take precautions, they are putting themselves and others at risk.
"Individuals have to take responsibility for their own behaviour, and not take the risks that they are taking at the moment."
Professor Troop said sex education needed to be more substantial, and open debate about sexual issues should be encouraged.
Open discussion
"We are not giving young people the opportunity to discuss some of these issues so that they can handle the relationships, and the problems they are going to encounter."
The chief executive of the Brook Advisory Centres group, Jan Barlow, said there was evidence to suggest young people were more aware of STIs, particularly for common infections such as chlamydia.
HAVE YOUR SAY
There needs to be a shift back to the days when parents and teachers told their kids its wrong to have sex outside of marriage
Richard, UK
But she said there was a long way to go, particularly in educating young people.
"We need to keep getting the message out that basically anyone who has unprotected sex is putting themselves at risk of infection."
She said it was as still a common perception that STIs "happen to other people and not to them".
Brooke is calling for sex and relationships education to be made a compulsory part of the National Curriculum in schools.
Melanie Johnson, the public health minister, said the government had recently invested £26m in sexual health services, and welcomed the slow down in the rate of increase of cases.
No room for complacency
She also highlighted the fact that the government had introduced a national screening programme for chlamydia.
But she said: "We are not complacent about this. We think there is a lot more to be done."
Mr James Johnson, British Medical Association chairman, said the figures made for "very depressing reading".
He said: "The BMA is particularly concerned that the increasing incidence of sexually transmitted infections is leaving genitourinary medicine (GUM) clinics unable to cope.
"Many patients are not being seen within the 48-hour target but only after weeks of delay.
"It is a scandal that the service we offer patients today is worse than it was 90 years ago. During the First World War a free, rapid and totally confidential service was set up."
The Family Planning Association said that waiting times for specialist sexual health clinics could be as long as six weeks, making the risk of passing on infections even greater.
A survey by the organisation found that 54% of the UK's 256 GUM clinics had opening times of less that 21 hours a week, meaning people had difficulty accessing services.
Tuesday's figures come ahead of a report by the Chief Medical Officer Sir Liam Donaldson, which is expected to reveal concerns about the number of undetected cases of HIV.
Sexually Transmitted Infections
New cases reported in England, Wales and Northern Ireland
Disease Cases 2002 Cases 2003 Change
Chlamydia 82,558 89,818 9%
Genital herpes 18,432 17,990 -2%
Genital warts 69,569 70,883 2%
Gonorrhoea 25,065 24,309 -3%
Syphilis 1,232 1,575 28%
Source: Health Protection Agency
