Sexually transmitted infections (STIs) in Britons aged over 45 has more than doubled in less than a decade with Internet dating and drugs that counter erectile dysfunction partly to blame, a report said on Monday.

Researchers from the government's Health Protection Agency looked at the number of patients visiting 19 sexual health clinics in central England between 1996 and 2003.

Although the vast majority of the patients were young, they found that in 1996, 344 people over 45 were diagnosed with an STI. By 2003, that number had risen to 780.

Men in the 55-59 age bracket were more likely to have an STI than anyone else, while rates were highest in women aged 45-54, said the study published in the Sexually Transmitted Infections magazine.

The most commonly diagnosed infections were genital warts, which accounted for almost half of cases, and herpes while the number infected with chlamydia, gonorrhoea and syphilis also rose sharply.

While the number of STIs among younger people rose 97 percent during the period studied, it increased by 127 percent among the over 45s.

The report suggested that changes in social and behavioural patterns were partly responsible, with older people less likely to use protection, which they associated with preventing unwanted pregnancy.

"There is also growing evidence that the Internet is being used to identify casual sexual partners by all age groups," it said, adding that meeting partners online was linked to an increased risk of acquiring an STI.

"It is also recognised that the introduction of drugs to counter erectile dysfunction has altered the quality of life and sexual experience of older individuals."

The researchers argued that health officials had focused almost exclusively on the sexual health of young people and that older age groups had been ignored.

They said that as people with more liberal sexual attitudes got older, the situation was likely to get worse.

The Family Planning Association (FPA) said it had noticed a rise in the number of people over 45 contacting its helpline.

"Tragically, the sexual health of men and women of this age group is largely neglected and it's something FPA is increasingly concerned about," said Julie Bentley, its chief executive.

"Once the worry of pregnancy goes away, it's easy to forget about sexually transmitted infections and the importance of using condoms."