This finding contradicts previous research, which suggested regularly smoking the drug could lead to adult substance abuse but it was less harmful if used infrequently.

Lead author Professor Louisa Degenhardt, of the National Drug and Alcohol Research Centre at the University of New South Wales, said the new study highlighted the need for early intervention to prevent children taking up the habit.

''The reason why this is important is because most young people only ever engage in occasional cannabis use,'' Professor Degenhardt said. ''What it definitely says is that early onset occasional cannabis use is a marker for being more likely to be engaging in a whole range of drug use behaviours in young adulthood.''

Joe Tucci, chief executive of the Australian Childhood Foundation, urged parents not to allow their children to experiment with cannabis.

''Parents need to realise that even occasional use of so-called recreational drugs is really the first step in a slippery slope. Patterns of behaviour start early in children, so these habits can be very hard to break,'' Dr Tucci said. ''[Parents] need to monitor their children's behaviour and monitor whether they're using even so-called 'soft' drugs. Cannabis can cause lots of detrimental impacts all the way through to psychosis as you get older, so the perception of cannabis as a softer, harmless drug is not right.''

Study participants were picked from Victorian private and public schools between 1992 and 1993 and were canvassed with computer and telephone surveys. They were followed up eight times over 10 years.

Professor Degenhardt called for caution in interpreting the study's results, saying it was unclear if cannabis was a ''gateway'' drug.

''Where you're talking about drugs that are not legal, if you're using one illegal drug you're probably going to have greater opportunity to use and know more people who are using other illicit drugs as well.''