So how dangerous is skunk?

The UN called the move courageous. Mental health experts described it as a crucial intervention. Sir Richard Branson says it is time to think again. And on pro-marijuana blogs and chatrooms from Sydney to Sarajevo, the mood was more murderous than mellow.

Seven days after The Independent on Sunday published evidence that almost 10,000 under-18s needed drug rehabilitation for cannabis use in Britain last year, a ferocious debate has begun over whether a potent new form of the drug is leading to growing mental health problems.

Beneath the headline: Cannabis - An Apology, the IoS revealed last week that more than 22,000 people were treated last year by the NHS for addiction and psychological problems caused by smoking skunk, which is up to 10 times stronger than resin or grass.

Ten years after Rosie Boycott, this newspaper's then editor, launched a campaign to have marijuana legalised, the IoS last week reversed its stance, stating that the evidence of mental health problems among smokers made the current 'C' classification correct.

The move has provoked an international debate about the threat posed by cannabis in general and skunk in particular. The warnings about the effects on mental health were underlined yesterday in a new report, saying almost a quarter of all new cases of schizophrenia would stem from cannabis smoking by 2010.

The study, published in the journal Addiction, also predicts that young men who smoke cannabis will be particularly at risk. The use of cannabis among under-18s rose 18-fold in the 30 years to 2002, according to the researchers.

Dr John MacLeod, co-author of the study, said: "If you assume a link [with cannabis] then the number of cases of schizophrenia will increase in line with increased use of the drug." Antonio Maria Costa, the executive director of the United Nations office on drugs and crime, yesterday called for people to wake up to the fact that cannabis is now a dangerous drug wrongly portrayed as a "gentle, harmless substance".

Writing in today's IoS, Mr Costa says that is no longer the case as a result of the potency of skunk, adding: "The cannabis now in circulation is many times more powerful than the weed which today's ageing baby-boomers smoked in college. Evidence of the damage to mental health caused by cannabis use - from loss of concentration to paranoia, aggressiveness and outright psychosis - is mounting and cannot be ignored."

Sir Richard Branson, head of the Virgin Group, who was a supporter of the IoS decriminalisation campaign, said yesterday: "The arrival of genetically engineered skunk has merited a new look at the situation."

Medical experts are now warning that the addictive nature of the drug means that detox clinics are needed. A report published in The Lancet last week showed how cannabis is more harmful than drugs such as LSD and ecstasy - but less so than alcohol and tobacco.