"Pharmaceologists and psychiatrists tell us that cannabis can have very severe short-term effects," he said. "The long-term effects include a very severe exasperation of mental illness and cancer. It is reckoned that cannabis is between two and four times as carcinogenic as tobacco.

"If cannabis were legalised, then consumption of a drug for which the evidence is very strong that it is very harmful will unquestionably increase. In five or ten years people will say: 'Why have you done this? It has made us more unhealthy."

"While it is undoubtedly the case that many drug addicts started with cannabis, to claim that taking cannabis is bound to lead to hard drugs has always seemed to me far-fetched."