Cannabis resin, usually smuggled into Britain from North Africa, has been replaced on the streets by home-grown skunk, a highly potent form of cannabis leaves. The plants can be grown in houses with lights and watering systems and produce a drug up to 25 times stronger than that available a decade ago.

Figures from the National Treatment Agency show that the number receiving treatment for addiction to cannabis has risen from 13,400 in 2004-2005 (excluding the North West) to 22,500 for the first 10 months of 2006-2007.

In that time the number of young people receiving treatment almost doubled from 5,000 to 9,500.

There has also been an increase in the number of patients treated in NHS hospitals for mental illness and behavioural problems linked to cannabis – from 581 in 2001 to nearly 1,000 last year.

The rise in those needing treatment has come as figures from the National Crime Survey show actual use of cannabis is falling.

Research has suggested that heavy use of cannabis is associated with a four-fold increase in the risk of developing schizophrenia. It is also associated with higher rates of depression and anxiety.

Prof Robin Murray of the Institute of Psychiatry said recently that 80 per cent of the patients he assessed with their first episode of psychosis said they had been taking cannabis. The institute is trying to work out whether mental illness is caused by taking cannabis or if mentally ill people are more likely to take cannabis to help them cope with their symptoms.

An independent drug policy commission, chaired by Dame Ruth Runciman, is to assess the Government’s approach to the problem.

Cannabis was downgraded from a class "B" to a class "C" drug in 2004 and a YouGov survey last year for The Daily Telegraph and the Royal Society for the Encouragement of Arts, Manufactures and Commerce (RSA) found respondents placed tobacco and alcohol well ahead of cannabis in substances thought to do most harm.

Via The Telegraph