Sexually transmitted diseases have doubled in less than a decade among a group of over-45 British residents, leading researchers to urge health officials to do more to dispel false assumptions about sex among older people.

"Although it is recognized that young people should remain the focus of sexual health programs, the results indicate that sexual risk-taking behaviour is not confined to young people, but also occurs among older people," say epidemiologists with the government's Health Protection Agency in the West Midlands.

Their research, published online Monday in the journal Sexually Transmitted Infections, tallied the numbers of STDs diagnosed in 19 sexual health clinics in the West Midlands between 1996 and 2003.

While the numbers of infections identified in older age groups are still relatively small compared to younger people, the epidemiologists found an increase among those over 45 of 127 per cent, compared to 97 per cent for those under 45.

In total, researchers found 4,445 cases in over-45, largely among heterosexual men and women. The most commonly diagnosed infection was genital warts, accounting for almost half. Herpes was the next most common, accounting for almost one in five.

Men and those between 55 and 59 were significantly more likely to have an STD than anyone else over 45. Among women, rates were highest among those 45 to 54; among men, rates were highest among those 55 to 60-plus.

The researchers said they were especially worried about the increase because other studies have shown it takes longer for older people to seek medical help after realizing they have a STD.

"Delay in accessing services can therefore lead to delayed diagnosis and delayed treatment, thereby increasing the potential for the development of complications in the infected person as well as the continued transmission of infection to sexual partners," they wrote.