A licence to smoke cannabis legally was proposed on Tuesday by one of Britain's leading experts on the drug.

Professor Roger Pertwee, of the University of Aberdeen, said making cannabis as available as alcohol would prevent drug-related crime and reduce the chances of people being introduced to harder narcotics.

However, the expert - who pioneered early research on the effects of cannabis in the 1960s and 1970s - cautioned that it might be necessary to prevent vulnerable individuals obtaining the drug.

He said: "You'd need to have a minimum age of 21, and I would suggest you might even have to have a licence You have a car licence and a dog licence; why not a cannabis licence?"

The idea would mean only those not suffering from a serious mental illness or at risk of psychosis would be legally allowed to buy the drug.

Research has shown an association between smoking cannabis and a greater chance of some individuals developing schizophrenia.

Prof Pertwee said cannabis appeared to increase the risk of psychosis in people already predisposed to the illness because of their genes or traumatic childhood.

He called for a greater debate on the recreational use of cannabis, and said in principle he was in favour of legalisation, if the right framework could be found.

"We need to explore all the various options. At the moment cannabis is in the hands of the criminals, and I think it's crazy. We're allowed to drink alcohol and smoke cigarettes. Cannabis, if it's handled properly, I think is no more dangerous than that."

He pointed out that currently anyone wanting to take cannabis was forced to grow it illegally or buy it from illegal dealers. The drug was supplied with no indication of what it contained, or what might have been added to it.

People also tended to smoke cannabis in groups, which increased the likelihood of psychological dependency. Licensed suppliers of the drug would also be less likely to provide a "gateway" to harder, more dangerous drugs.

Prof Pertwee added: "I think this could be the way forward, but it might not work. It depends on a private company being willing to produce a branded product."

Prof Pertwee is speaking at the British Festival of Science at Aston University, Birmingham, this week.