Charles Clarke said today that cannabis would not be reclassified as a Class B drug again, despite conceding a link with mental illness and labelling the substance "anything but harmless".

While pledging to launch a public information campaign on the dangers of cannabis use, Mr Clarke said that reclassification had not led to an increase in use.

David Blunkett, his predecessor as Home Secretary, downgraded cannabis from a Class B to a Class C drug in 2001.

Mr Clarke told MPs that he had been guided by a report by the Advisory Council on the Misuse of Drugs (ACMD). "I have decided to accept the Advisory Council’s recommendation, which is supported by the police and by most drugs and mental health charities, to keep the current classification of cannabis," he said.

David Davis, the Shadow Home Secretary, said that he was "frankly disappointed" by the decision.

"I am frankly disappointed that, in the light of all the new evidence available, he has not decided to grasp the nettle and reclassify cannabis back to a B class drug.

"This ongoing confused message will lead some to continue thinking cannabis is a safe, 'soft', drug. It will mean that many more young lives will be damaged by the pernicious trade in this dangerous drug," he said.

Mr Clarke said that he accepted that cannabis could trigger or exacerbate serious mental health problems including schizophrenia.

He said that he means to overhaul the way drugs are classified and prohibited, after concern that the existing system - of Class A, B and C substances - had caused some people to misinterpret the downgrading of cannabis.

He also announced that the ACMD will carry out a new inquiry into the classification of "date-rape" drugs, including GHB and Rohypnol.

Concerns about a link between super-strength cannabis varieties and mental illness have mounted since the down-grading of the drug took effect in January 2004, making cannabis possession a non-arrestable offence in most cases.

Mr Clarke said that he will launch a major public information campaign to stress that cannabis was "anything but harmless".

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