TOBACCO barons have warned that they will invoke the freedom of speech clause under the Bill of Rights if moves are made to introduce a total ban on cigarette advertising. Cigarette industry leaders in Hong Kong said the clause gives them the right to advertise, as it safeguards freedom of information. The warning came in speeches by the chairman of the Tobacco Institute of Hong Kong, Brendan Brady, and the public affairs director of Rothmans Ltd, Robert Fletcher, at a Society of Hong Kong Publishers luncheon yesterday. Cigarette advertising on television and radio was banned two years ago and the tobacco bosses said any further restrictions would begin to impinge on ''commercial freedom of expression''. The Council on Smoking and Health, COSH, is backing a bid to ban all tobacco advertising by the year 2000. ''One of the ironies of freedom of speech is that some people may say things you don't like and with which you disagree. But you still have to defend their right to say them,'' Mr Fletcher said. The tobacco industry representatives said cigarette advertising did not encourage people to smoke. ''The whole issue is one of commercial freedom of speech for legal products,'' Mr Brady said. Angeline Oyang Ying-lan, the executive director of COSH, said: ''The idea that this advertising does not encourage people to smoke is a joke. ''If advertising didn't work then these companies would not bother to spend so much on it.''