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Regular cannabis users are at greater risk of developing mental illness later in life, according to research.

One study found that the risk was seven times higher for heavy users, said Professor Robin Murray of the Institute of Psychiatry in London.

Speaking at the Royal College of Psychiatrists' annual conference in Edinburgh, he said: "In the last 18 months a number of studies have confirmed that cannabis consumption acts to increase later risk of schizophrenia. This research must not be ignored."

The findings come as the Government prepares to downgrade cannabis from a Class B to Class C drug next year.

Most people caught in possession of a small amount will have the drugs confiscated and receive a reprimand or warning, the Home Office has said.

According to a Government fact sheet, cannabis "can cause psychotic reactions amongst individuals with mental health problems", but it does not suggest use of the drug can prompt those problems.

For his study, Professor Murray reviewed research in Sweden, Holland and New Zealand.

A recent Dutch study of 4,000 people in the general population showed that those taking large amounts of cannabis were almost seven times more likely to have psychotic symptoms three years later.

Another study, in 1987, of 50,000 Swedish Army conscripts, found that those who admitted at age 18 to having taken cannabis on more than 50 occasions, were six times more likely to develop schizophrenia in the following 15 years.

Professor Murray said these findings had been largely ignored

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