It could be something in the cocoa or too much titillation on Coronation Street: Britain's pensioners are shedding their cardigans and getting down to it.

But the men and women who began the sexual revolution as twentysomethings in the late 1950s are not only finding new passion and partners in their retirement, they are also responsible for a dramatic rise in sexually transmitted diseases.

People living longer and healthier lives, rising divorce rates, the absence of work or family-related stress, even Viagra, may be responsible for the new promiscuity of older people who seem to have missed two decades of public sexual health messages often targeted at the younger, more at-risk groups.

A charity is now calling on the Government to investigate and to create a new safe sex advertising campaign aimed at older people. 'This should come out of the closet,' said Tessa Harding, head of policy at Help the Aged. 'I think there's very little public or professional recognition of the sexual health of older people. It's not one of those topics people want to talk about.'

In America last week the Centers for Disease Control in Atlanta reported that the incidence of HIV in people aged over 50 is increasing at twice the rate for those younger.

Officials have speculated that a more open society, people entering the dating scene after the monogamy of a marriage and the absence of a fear of pregnancy is causing the alarming rise in sexually transmitted infections.

Grandparents behaving badly is not something either society or their kids want to think about. A north London care home worker told The Observer that 'residents being found entwined' was a not uncommon embarrassment for visitors while last year four elderly people had been asked to move out after being caught in 'a midnight orgy' in the dayroom.

'The idea that doctors shouldn't approach this question because it might be offensive is really passé,' said a CDC spokesman. 'If we don't change this predilection in physicians, we're essentially condemning elders with HIV to an early death. That's true more today than ever with the disease management tools we have.'

Dr Marcia Ory, chief researcher at the US National Institute on Ageing, and chair of an Aids and Ageing conference, held last week in Maryland, said: 'We can't ignore the older population, and have to understand the role they're playing in the HIV/Aids epidemic.' She said that 10 per cent of HIV cases were now in the elderly population.

In the UK, sexually transmitted infections have increased across the board from 624,000 in 1990 to almost one and a quarter million in 1999 - the most recent figures.

While in the whole population rates of gonorrhoea and syphilis have increased by 55 per cent since 1995, in the 65-plus age group the rise is more than 300 per cent.

But the rise could possibly mean that the stigma of attending an STI clinic is eroding, according to veteran agony aunt and social campaigner Claire Rayner.

'Years ago an older man who caught a dose of the clap from a prostitute would not be contacting his doctor. People were stoical and not over keen to present themselves for treatment but now more people are prepared to seek help.

'People do insist in thinking that older people are asexual. Believe me they are not, they have been having a randy time for a long time.

'The male menopause panic has always made men run out and find a younger woman to prove they can still do it and now women are doing that too. Safe sex has always for them been about avoiding pregnancy not avoiding infection so once that fear is gone they throw caution to the wind.'

The Department of Health is compiling a strategy for tackling the STI issue across the board. 'Sexual health is not just an issue for young people, it's an issue for people of all ages,' she said. 'More of our proactive work is focused on younger people because we know that is the biggest at-risk group, but we are presently compiling a report on sexual health which will give us the clearest picture to date of what is happening. We will be looking to see how relevant the rise in STIs in the older age group might be.'