Mental health charity SANE has called on the government to consider the long-term risks of cannabis in deciding whether to reclassify it as a class B drug.

Chief executive Marjorie Wallace said: 'Although many can smoke a joint with no long-term effects, it is estimated that 10 per cent of people who regularly use the drug could be at risk of developing this serious condition, in which a person may hear voices and experience strange thoughts and paranoid delusions.'

She added: "Any decision regarding the drug's classification has to balance the medical and recreational benefits against the devastation to the lives of the 800 people each year who, without cannabis, would not have developed psychotic illness."