Cannabis is to be reclassified from a class B to a class C drug, despite evidence that cannabis use by teenagers is a major cause of psychosis.

The government has announced a £1m information campaign about the law change, with radio advertisements targeted at young people telling them that cannabis remains illegal, and that under-18s can still be arrested for possession.

But reclassification means that possession for over-18s will no longer be an arrestable offence except in "aggravated" cases such as smoking cannabis near children.

Home Office drugs minister Caroline Flint said "by reclassifying cannabis we are being honest to young people about the harm cannabis can cause in comparison to drugs such as crack and heroin. This is an open and effective way to tackle the problems associated with class A drugs."

Cannabis will be reclassified on 29 January as party of the government's overall drug strategy. The change was recommended by an independent expert committee, the Advisory Council on the Misuse of Drugs, on the grounds that heroin and crack cocaine are more harmful to people.

But the British Medical Association has warned that the public may be misled into believing cannabis use is safe. "We are very worried about the negative health effects of smoking cannabis and want the government to fund more research on this issue.

Research published by the Institute of Psychiatry warned that regular cannabis use among teens can cause psychotic illness.

In an interview with The Times, professor Robin Murray, the author of the study, suggested the government should think again about reclassifying cannabis.

"Unfortunately there were no experts in psychosis on the committees (the Home Affairs select committee and the Advisory Council on the Misuse of Drugs) that advised the government on reclassifying cannabis. That's not a criticism - at the time, no one thought there should have been. Since then there have been at least four studies that show the use of cannabis can significantly increase the likelihood of the onset of psychosis.

"I would say this is now the number one problem facing the mental health services in inner cities. In south London the incidence of psychosis has doubled since 1964. There is a terrible drain on resources. Not only are there people suffering from psychosis who would not be in in-patient beds if they were not using cannabis, but use of the drug also drastically reduces the chances of recovery.

"People who do improve go out on the streets, meet their old dealer, begin using the drug again and relapse. We're not saying that the government shouldn't reclassify cannabis - for most people it causes no problems - but I am saying that if they're going to do it they should warn people of the possible downside." 

- For more information go to www.mentalhealthcare.org.uk/schizophrenia/research/cannabis/