By the end of the decade one in four new cases of schizophrenia could be triggered by smoking cannabis, scientists say.

Research has suggested that regular users of the drug are up to six times more likely to develop schizophrenia, although whether the drug is the direct cause remains disputed.

The Department of Health says it is now generally agreed among doctors that cannabis is an “important causal factor” in mental illness.

A study published in the journal Addiction predicts that, if the causal link between cannabis use and schizophrenia is accepted, rates of the illness will increase substantially by 2010, especially among young men.

The use of cannabis grew fourfold over the 30 years to 2002, and eighteenfold among under18s, the researchers say.

Such a boom in cannabis use could lead to increases in the number of new cases of schizophrenia of 29 per cent between 1990 and 2010.

Cannabis was downgraded from a class B drug to class C in 2004. The new study comes after a new classification system for drugs proposed by scientists this week placed tobacco and alcohol well ahead of cannabis in substances thought to do most harm.

The researchers examined the historic rise in cannabis use alongside new occurrences of schizophrenia in Nottingham, Bristol and the London Borough of Southwark. Predictions based on the data suggest that if the association is confined to heavy cannabis users then approximately 10 per cent of schizophrenia cases may be due to cannabis by 2010.

However, assuming an association between onset of the disease and both light and heavy users, then as many as one quarter of new cases could be due to cannabis.

So far, about 8 per cent of schizophrenia cases have been attributed to the use of cannabis.

John Macleod, a co-author of the study and an academic GP, said: “Our study does not address the question whether cannabis directly causes schizophrenia — this remains unclear — but what is clear is that if you assume such a link then the number of cases of schizophrenia will increase significantly in line with increased use of the drug.”

Matthew Hickman, the lead author of the study, added: “The challenge now is to improve our data on schizophrenia occurrence to see whether the projected increase occurs — which will tell us more about how important cannabis is as a cause of schizophrenia.”

According to the mental health charity Rethink, at least five international studies have found that cannabis doubles the risk of mental illness, including schizophrenia.

A spokesman for the charity said: “We now know that cannabis can be a trigger for mental health problems and smoking it under the age of 18 can double people’s chances of developing psychosis.”

Last year the Advisory Council on the Misuse of Drugs concluded that there may be a link between cannabis use and psychotic illnesses, but found insufficient evidence that frequent users were more likely to develop schizophrenia.

There are also fears that modern variants of the drug such as skunk, from the tips of the cannabis plant, are 20 to 30 times more powerful than those available 30 years ago.

However, Leslie Iversen, of the University of Oxford, dismissed this as untrue. “The advisory council looked at this carefully,” he said. “Cannabis resin has changed little and is about 5 per cent tetrahydrocan-nabinol [THC]. Skunk has 10 to 15 per cent THC. That makes it two to three times more powerful, not 20 to 30 times.”

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