Casual sex is fuelling a big rise in sexual disease among young people, health experts warned today, as figures showed a 6% increase in infections.

Young people, aged 16 to 24, accounted for half of all newly diagnosed sexually transmitted infections (STIs) in 2007, though they make up only one-eighth of the population.

Infections rose across the UK, figures from the Health Protection Agency (HPA) show.

Professor Peter Borriello, the director of the HPA centre for infections, blamed the rise among young people on the prevalence of unsafe sex.

"It's increasingly the case that among young people a casual shag is part of the territory, it's part of life," he said.

"Increasingly a shag now stands for syphilis, herpes, anal warts and gonorrhoea.

"If you are going to go swimming, dive into the pool, make sure you know how to swim, be safe. That really means wear a condom."

There were 397,990 newly diagnosed STIs in clinics last year, up from 375,843 in 2006.

New cases of genital herpes rose 20% while there was a 7% rise in genital warts and chlamydia.

Young people accounted for 65% of all chlamydia, 50% of genital warts and 50% of gonorrhoea infections diagnosed in genitourinary medicine clinics across the UK last year, the HPA said.

This is the eleventh year in a row that STIs have risen year-on-year.

And young people are not the only group affected. Sexually transmitted infections among people over 45 have doubled in under a decade, a recent study showed.

The HPA said increases in testing and diagnoses accounted for some of the rise.

But Borriello called for a strong message to be delivered to those who engage in casual encounters without taking proper precautions.

The HPA want sexually active young people to be screened for chlamydia annually and every time they change their sexual partner.

Chlamydia, which often has no symptoms, remains the most common STI in the UK.