LONDON (Reuters) - Young sexually active people should get themselves tested annually for the infection chlamydia and again every time they change partner, the Health Protection Agency said on Tuesday.

Young people were disproportionately affected, said Peter Borriello, Director of the HPA's Centre for Infections.

"Substantial numbers of young people remain undiagnosed, untreated and unaware of the risk they pose both to their own health and that of their sexual partner," he added.

People aged 18-24 form just one in eight of the population but account for around half of all newly diagnosed sexually transmitted infections (STIs) in the UK.

This age group suffered 65 percent of all chlamydia cases, 55 percent of all genital warts and 50 percent of gonorrhoeae infections diagnosed in genito-urinary clinics last year.

The total number of STIs diagnosed reached 397,990 in 2007, up from 375,843 the year before.

Borriello said one reason for the rise could be an increase in the number being screened.

More than one million sexual health screens were conducted in 2007, ten percent more than in 2006.

"If sustained, this could have a significant impact on the control of sexually transmitted infections," said Borriello.

"However, we cannot rely on prompt diagnosis and treatment alone -- a shift in behavior is the only way that we will bring down this continued increase in infections."

Nearly one in 10 sexually active young women tested by a chlamydia screening programme in England last year were found to have the infection, which can cause pelvic inflammatory disease, ectopic pregnancy and infertility.

One in 12 men was also found to be carrying the disease, the world's most common sexually transmitted infection.

A study by University College London last November showed that one in five young Britons has sex with a new partner when traveling abroad.

(Reporting by Tim Castle; Editing by Steve Addison)