KEY POINTS:
Britain's Prime Minister Gordon Brown is this week expected to reject the opinion of the Government's own Advisory Council on the Misuse of Drugs (ACMD) and signal that cannabis should be reclassified from a class C to a class B drug.
The move is a spectacular U-turn from just two years ago and the first time ministers have ignored their experts since 1971.
It means those caught in possession of cannabis could face prison sentences of up to five years, compared with two now. But the change, for the majority of users, is likely to be cosmetic. The police have signalled that they will still continue a policy of "confiscate and warn", although persistent offenders will face tougher penalties which experts believe will see more ending up in prison.
Last year police warnings on cannabis rose 20 per cent to 120,000, suggesting the new approach is proving popular with officers on the streets, as it frees them from the bureaucracy associated with making arrests.
Mr Brown's supporters insist the reclassification will send a signal to society, particularly young people.
But the shift is seen by some as Mr Brown exerting his authority as a conviction politician, rather than listening to the experts. Few in the field believe the move is justified or indeed will have any effect. In 2006, the House of Commons Science and Technology Committee found "no solid evidence" that classification had a deterrent effect on consumption.
Only 3 per cent of people polled by the mental health charity Rethink said a change in classification would deter them from smoking cannabis.
Cannabis is now a big black-market business in Britain. While heroin is imported from the east, cocaine from South America and ecstasy from the Netherlands, much of the cannabis crop is homegrown.
Sir Stephen Lander, the head of the Serious and Organised Crime Agency, warned earlier this year that large-scale cannabis factories - producing high-strength strains of the drug and run by Vietnamese and Chinese criminals - are appearing across the country. Charities working with immigrant communities claim that some factories rely on smuggled child labour to maintain the plants. What was a cottage industry has become an industrialised cultivation.
Soaring demand for the stronger strains of cannabis is reflected by police seizures. The Home Office has been quietly studying the results of a survey conducted among police forces across the UK.
Early findings suggest sinsemilla - the potent herbal leafy variety of cannabis made from dried seedless female plants, of which skunk is just one of about 100 strains - accounts for 80 per cent of all cannabis seizures.
The crucial issue dividing politicians and mental-health experts, though, is whether this polarisation of cannabis is having a deleterious effect on the nation's mental health.
"We have been campaigning for many years about the links between cannabis and psychiatric illness, and highlighting evidence that the drug may not only precipitate psychotic breakdown but cause long-term mental damage," says Marjorie Wallace, chief executive of the mental health charity Sane.
A problem for the anti-cannabis camp, however, is that cases of psychosis have actually been falling. Research presented to the ACMD showed incidents of schizophrenia declined between 1996 and 2005.
Government surveys also show that cannabis use in younger people is declining.
SMOKE SIGNALS
* A British Medical Journal study found those using cannabis before the age of 15 are four times as likely to develop psychotic illness by 26.
* A Lancet study in 2007 estimated that 14 per cent of 15 to 34-year-olds affected by schizophrenia are ill because of heavy cannabis use.
* Recent analysis of 35 major studies concluded that cannabis use increased the risk of psychotic illness later in life by approximately 40 per cent and by up to 200 per cent among heavy users.
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