Cannabis users are 40 per cent more likely to develop a psychotic illness than non-users, a study has found.

Heavy users are more than twice as likely to suffer mental illness, according to a group of British academics, who calculate that about one in seven cases of conditions such as schizophrenia is caused by cannabis.

The warnings come as the Prime Minister and the Home Secretary signalled that the “softly softly” era for cannabis may be coming to an end.

Gordon Brown said last week that the Home Office would be consulting on whether it had been right to downgrade cannabis from a Class B to a Class C drug in 2004. Jacqui Smith, the Home Secretary, is to ask the Advisory Council on the Misuse of Drugs to review the evidence.

The paper, published in The Lancet, is written by a group of seven psychiatrists and psychologists from Bristol, Cardiff, London and Cambridge.

They have pooled the findings from 35 studies in a number of countries, including the United States, Germany, the Netherlands, Sweden and Britain, and concluded that there is “a consistent association between cannabis use and psychotic symptoms, including disabling psychotic disorders”.

They admit that they cannot be certain that the association means that there is a simple cause and effect, but say that policymakers “need to provide the public with advice about this widely used drug”. They go on: “We believe there is now enough evidence to inform people that using cannabis could increase their risk of developing a psychotic illness later in life.”

As well as looking at psychotic illness, they looked for evidence that cannabis could cause affective disorders such as depression, anxiety and suicidal thoughts. Almost all the studies point towards an increased incidence of such disorders. The evidence is less strong, the writers say, but is still of concern.

The study was welcomed by many experts, but others counselled caution. Leslie Iverson, of the University of Oxford, a member of the advisory council, said: “Despite a thorough review the authors admit that there is no conclusive evidence that cannabis use causes psychotic illness. Their prediction that 14 per cent of psychotic outcomes in young adults in the UK may be due to cannabis use is not supported by the fact that the incidence of schizophrenia has not shown any significant change in the past 30 years.”

But Robin Murray, of the Institute of Psychiatry at King’s College London, called it “a very competent and conservative assessment of what research studies tell us about the relationship between cannabis and psychiatric disorders”.

He said that the risk could be even higher then the authors had estimated, because the cannabis available today was stronger than in the past. “This report cannot tell us whether the risk is higher with the use of the skunk-like preparations which are now widely available, and which contain a higher percentage of tetrahydrocannabinol,” he said. “My own experience suggests to me that the risk with skunk is higher. Therefore, their estimate that 14 per cent of cases of schizophrenia in the UK are due to cannabis is now probably an understatement.”

Martin Barnes, chief executive of Drugscope and also a member of the council, said: “Cannabis is not harmless, and although it has been known for some time that the drug can worsen existing mental health problems, it may also trigger the onset of problems in some people.”

“The challenge is to ensure that information on cannabis use and the associated risks is understood by teachers and health professionals working with young people and conveyed in ways that young people will listen to. Since reclassification, cannabis use has continued to fall. We need to make sure this trend continues.”

Marjorie Wallace, chief executive of the mental health charity SANE, said: “The Lancet report justifies SANE’s campaign that downgrading a substance with such known dangers masked the mounting evidence of direct links between the use of cannabis and later psychotic illness. The debate about classification should not founder on statistics but take into account the potential damage to hundreds of people who without cannabis would not develop mental illness.

“While the majority can take the drug with no mind-altering effects, it is estimated that 10 per cent are at risk. You only need to see one person whose mind has been altered and life irreparably damaged, or talk to their family, to realise that the headlines are not scaremongering but reflect a daily, and preventable, tragedy.”

Martin Blakeborough, director of the Kaleidescope Project and a member of the council, said that it would be a waste of public money for the same panel, with the same evidence, to review the issue again. “There is significant danger in reviewing cannabis again, as it takes experts’ minds off more important issues. Classification itself, although important, is not as urgent as the increasing epidemic of hepatitis B and C among drug users and the wider community, or the increase of stimulant drugs in our community.”

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