About a fifth of Thailand's 60m people smoke and more than 80,000 Thais die each year of lung cancer.

Dr Prakit Vateesatogkit: Women are worried about their husbands

The doctor spearheading the country's anti-smoking campaign, Dr Prakit Vateesatogkit, said Thailand was the first country to introduce the warning.

But the new message has prompted a torrent of worried callers - mainly from women.

"They want their husbands to stop smoking," Dr Prakit said. "If smoking affects their husband's sex life, then maybe they will convince them to stop."

He said the new slogan already appears to have had more impact than earlier warnings like "Smoking causes lung cancer" or "Smoking makes you look older than your years".

Smoking doubles risk of impotence

Dr Prakit said the warning was backed by research by the American Council on Science and Health which had linked smoking to impotence.

A 1996 report said there was "considerable evidence that smoking decreases sperm production, deforms sperm, changes sperm mobility and seriously reduces blood flow to the penis, in some instances causing impotence".

"If you are non-smoker you have X amount of chance of becoming impotent," said Dr Prakit. "If you smoke you double that figure.

"If you want to take the chance, it's up to you, but as doctors, we have to tell you the facts.''

Thailand added the warning to a list of nine existing health messages which must cover at least 30% of the cigarette packet surface.

The other nine stress concerns ranging from "Cigarettes affect passive smokers" to "Smoking leads people to other addictive substances."