Many experts now believe that use of the drug can worsen symptoms in someone who already has schizophrenia, or manic depression associated with psychotic symptoms and it is thought that cannabis may act to switch on key genes related to such conditions in certain vulnerable people.

It is also believed by some researchers that long-term use of cannabis, particularly when use begins in early teens, can trigger schizophrenia in people who should be at no or low risk of developing the illness.

A study from New Zealand published last year found that smoking cannabis virtually doubles the risk of developing mental illnesses such as schizophrenia and another, by Swedish researchers, suggested cannabis use increases the risk of schizophrenia by 30%.

And the mental health charity Mind reported that psychiatrists had seen an increase in numbers of people being admitted to hospital with psychotic episodes linked to cannabis use since the drug was downgraded in the UK.

Because the new forms of cannabis produced by hybridisation and modern horticultural techniques are a lot more potent now, researchers have become concerned that many of the studies into the effect of the drug may be out of date.