Young people should be warned that smoking cannabis can increase the risk of developing psychotic illnesses such as schizophrenia by more than 40 per cent, researchers say.

The risk is doubled among regular users who smoke the drug daily or weekly, according to the review of 35 studies published in The Lancet.

The findings will add to pressure for the drug to be re-classified from class C to class B, which carries tougher penalties, in a review, promised by Gordon Brown, of the 2004 decision to downgrade it.

But the research was challenged by experts who said that there had been no increase in schizophrenia over the past 40 years, despite the explosion in cannabis use by young people. An estimated 6.2 million people take the drug.

The researchers led by Stanley Zammitt of Cardiff University say they cannot prove cannabis causes schizophrenia but the association is strong enough to warn young people of the dangers.

The findings imply that the lifetime risk of developing schizophrenia of 1 per cent in the general population would be raised to 2-3 per cent among regular cannabis users.

Overall about 14 per cent of "psychotic outcomes" might be avoided if cannabis were not consumed, the researchers say, equivalent to 800 cases a year. This is double previous estimates, which have suggested 7 per cent of cases of schizophrenia and associated illnesses might be attributable to cannabis.

Dr Zammit said: "It is not very helpful to mix the debate about the legal classification with the study findings. There is a need to educate people that there are these risks, and regular users ought to consider cutting down or stopping."

Professor Leslie Iversen, a pharmacologist at Oxford University and member of the Advisory Council on the Misuse of Drugs, said there were "all sorts of confounding factors" that could explain the association. "There hasn't been an upward blip [in schizophrenia] of any kind since cannabis use became common in the 1960s and 1970s."