Cases of sexually transmitted infections (STIs) in the UK rose sharply again last year because many people wrongly consider they are trivial diseases and have unprotected sex, health experts warned today.

There were around 700,000 new diagnoses of STIs in 2004 - a rise of more than 60% since 1995, according to the Health Protection Agency (HPA), which monitors infectious diseases.

The HPA found the biggest rises in infection rates over 2003-04 were seen in cases of chlamydia, syphilis and genital warts.

The number of people diagnosed with syphilis rose by 37% from 1,641 to 2,254. New diagnoses of genital warts rose by 4.2%, from 76,457 to 79,678.

But chlamydia remains the most common STI, with the number of new cases rising by 8.6%, from 95,879 in 2003 to 104,155 in 2004. The biggest increase was seen in heterosexual men, where cases went up by 13%, while in women they rose by 5.5%.

Since 1995 the incidence of chlamydia, which often has no symptoms and can cause infertility, has risen by 223%.

Professor Peter Borriello, director of the HPA's centre for infections, expressed concern that many people considered STIs as "a little bit trivial". He said: "We are concerned that the messages that we are trying to get to people may not be hitting home as hard as they should. There may be the risk of message fatigue."

However, there was an 11% decrease in diagnoses of gonorrhoea, from 24,956 in 2003 to 22,335 last year. There was also a 1% decrease in diagnoses of genital herpes, from 19,180 to 18,991, over the same period.

The HPA report, published to coincide with World Aids Day, also noted that there were an estimated 58,300 HIV positive people in the UK last year - 5,300 more than in 2003.

The agency believes that more than a third (34%) of those it estimates to be HIV positive - around 19,700 people - are undiagnosed, meaning they could unknowingly be spreading the virus that causes Aids.

Its report, Mapping the Issues, states there were 7,275 people newly diagnosed with HIV last year, compared to 7,217 in 2003. More than half the new cases last year - 4,287 - were diagnosed in heterosexuals, with 73% of those most likely to have been contracted in Africa.

In contrast, three-quarters of cases contracted in the UK were in gay and bisexual men. The total number of all new diagnoses in this group last year was 2,185 - the highest since 1990. The HPA estimates that 29,700 of those with HIV in the UK are heterosexual. There are also thought to be 1,650 under-15s with HIV.