The Advisory Council on the Misuse of Drugs will meet this week to decide whether to review new evidence suggesting cannabis can cause mental illness.

Before the election Charles Clarke asked the committee to reassess the government’s decision 16 months ago to downgrade crimes involving cannabis. Both Clarke and Tony Blair are understood to regret the decision, which coincided with an influx of stronger strains of the drug to Britain.

However, a leading member of the committee said last week he would be “very surprised” if it decided to urge a reversal.The Rev Martin Blakeborough, who runs the Kaleidoscope drug abuse charity in Kingston, west London, said the committee had already made its decision when it recommended in 2001 that penalties for using the drug be reclassified from category B to category C.

Blakeborough said there would need to be “an awful lot” of new evidence to convince the committee. “I would be extremely surprised if anything were to happen in terms of change,” he said.

Blakeborough added that senior police were in favour of the relaxed laws. Officers were issued with guidelines saying that possession in small quantities for personal use should no longer lead to an arrest. Arrests for cannabis possession halved in the first year of the relaxed regime, freeing up officers’ time to deal with other crimes.

Lord Adebowale, another committee member and chief executive of Turning Point, a drugs charity, is also said to be sceptical about tougher penalties. He has said any decision to review the drug’s status should be based on “clear, hard facts and not conjecture”.

However, Blair has told colleagues that he is “dead set” against the decision to downgrade the drug. Before the election he told parents there was increasing medical evidence that cannabis was “not quite as harmless as people make out”.

Concerns have also risen among mental health professionals. A study by the Institute of Psychiatry at King’s College London suggests that one in four people carries genes that increase vulnerability to psychotic illnesses if he or she smokes cannabis as a teenager.

Majorie Wallace, head of the mental health charity Sane, has warned that cannabis places millions of users at risk of lasting mental illness.

Some who supported downgrading cannabis are now reconsidering. Rosie Boycott, the former newspaper editor, wrote yesterday she had begun to have second thoughts after hearing of young people suffering mental illness after taking cannabis, particularly skunk, an extra-strong form of the drug.

In one case, told to her at a dinner party, “what was beyond doubt for these three boys was that skunk had caused a dramatic, sudden and very distressing change in their personalities”.

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