The ASA's ruling, which calls on the society to 'moderate' its presentation, came after a complaint by the Ministry of Agriculture. The society accused the ministry of being in league with the meat industry.
The leaflet, 400,000 copies of which have been distributed over four years without complaint, according to the society, is part of its School Campaign for Reaction Against Meat (SCREAM). It speaks of pigs kept in a 'degrading, squalid prison' and of bleeding to death in agony, and of 'fully conscious' chickens being boiled alive in scalding water. It goes on: 'Have you ever heard a pig cry out in terror? Have you smelt the stench of fear when an animal knows it is going to die?'
The complaint is understood to have been made to the ministry by a junior school teacher. The society said yesterday that the leaflet was issued only to children aged between 12 and 18.
Juliet Gellatley, the society's campaigns director, said the authority had withdrawn a suggestion that the leaflet 'exploited children's credulity' because it accepted that it was accurate.
References to pigs crying in terror, she said, came from witnesses in slaughterhouses. 'I would challenge anybody who does not believe that to go and watch a pig being slaughtered. If people are uncomfortable with that, I suggest they turn vegetarian.'
She added: 'The Ministry is totally tied up with the meat industry and the Meat and Livestock Commission, which it funds to the tune of pounds 15m a year. John Gummer (the Minister of Agriculture) is one of those traditionalists who seem convinced that vegetarians are abnormal. They don't like the fact that vegetarianism is increasingly so rapidly.'
Mr Gummer has described vegetarian diets as 'faddist' and unnatural and the commission has said the Vegetarian Society is a danger to democracy.
A Bradford University study last year indicated that there were 3.6 million vegetarians in Britain, 7 per cent of the population, compared with 1.5 million in 1985 and an estimated 100,000 in 1945. Ms Gellatley said the rise was linked with the growth in awareness of environmental issues and factory farming.
She said the society had taken part in hundreds of secondary school debates on animal welfare, at the request of teachers and usually in opposition to the commission: animal cruelty was the subject younger teenagers were 'most interested in'. The meat industry 'indundated' schools with material which ignored cruelty.
She described the complaint as a commercially-based attempt to stifle a truthful argument: the society would do 'absolutely nothing' about the ruling.