If swallowed, death is certain. There is no known antidote. It is often used in suicide attempts, according to the World Health Organisation, which rates it 'moderately hazardous'.

Michael Cheng Wan-kuen, herbicide and pesticide officer for the AFD, which controls pesticide use, said the paraquat sold in Hong Kong contained an emetic, to make someone who swallowed it throw up. It was also dyed and had a strong odour.

Mr Cheng said the measures were introduced to stop accidental poisoning. 'All herbicides and pesticides have potential risks. We did have a lot of concerns about paraquat which is why we added the extra controls . . . in September 1994.

'Society needs it, and there are not too many alternatives. But there is a risk if you do not use it properly or correctly.' Friends of the Earth has called for paraquat to be banned and for government departments to keep data on poisonings. 'What we think is that substances which are harmful or potentially harmful should be banned,' a spokesman said.

'When chemical companies assess toxicity they usually relate it to adults in a very healthy state - whereas the real effect on elderly people, children, or those in a weakened state can be much worse.' Paraquat is a growth hormone and attacks the vital organs, leading to death. Weeds are destroyed by the herbicide, but it can also remain in the soil, say environmentalists.

In a research paper, Friends of the Earth revealed traditional spraying methods showed only one to 10 per cent of a herbicide reached its target - the rest floated in the air and landed on one's hands, feet, face and body.

While birds, fish, frogs, snails and insects can die from consuming paraquat, and dozens of dogs in Hong Kong have died after eating substances contaminated with it, the chemical is on sale at about 400 agricultural and farm supplies shops and about 10 pet shops.