A growing craze for al fresco sex with strangers has led to a serious rise in sexually transmitted diseases, a health chief warned yesterday.

The practice, known as "dogging", where strangers gather to watch each other have sex outdoors, is causing so much concern for health officials in Kent that they have posted messages on the internet urging those involved to take better precautions.

Hundreds of semi-secluded sites around the country are advertised on websites as meeting places for Britain's doggers, who are understood to have more than doubled in number over the past two years.

One website, which lists dogging hotspots as well as providing a chat forum for aficionados, describes the practice as a combination of exhibitionism and voyeurism.

"Dogging is a broad term used to cover all the sexual outdoor activities that go on. This can be anything from putting on a show from your car, to a gangbang on a picnic table.

"The voyeurs are mainly men and the exhibitionists are mainly couples or women who love to attract attention and often invite people to join in," the host, Melanie, informs readers.

But as cases of HIV, chlamydia, syphilis, hepatitis and gonorrhoea continue to rise in certain parts of the county, Mathi Chandrakumar, clinical director of the Kent health protection unit, is urging participants to realise the risks involved.

"If people are having unprotected sex without knowing anything about the history of their partner then they are putting themselves in great danger.

"Towards the end of last year we noticed a rise in the number of hepatitis B cases. Instead of the usual one or two cases, there were more than a dozen cases over a six-month period.

"Investigations showed some of these cases were linked to the practice known as dogging," he said.

Messages have now been posted on a number of websites by the health promotion team, warning of the rise in sexually transmitted diseases in the area and urging people to "avoid the sex lottery" and play safe.