The home secretary, Charles Clarke, today ruled out another reclassification of cannabis despite recent warnings that the drug can cause serious mental illness.

Mr Clarke said cannabis would not be changed back to a class B drug, instead announcing a public health campaign to warn people of the health risks associated with its use.

He told MPs that his decision to keep cannabis as a class C drug had followed advice from the Advisory Council on the Misuse of Drugs, and was supported by police and most drug and mental health charities.

The council's unpublished report is said to have concluded that the risk of someone developing schizophrenia as a result of using cannabis was "very small".

It is believed to have said it was a "substantially less" harmful drug than those currently classified as class B, including amphetamines such as speed and barbiturates.

Cliff Prior, the chief executive of the mental health charity Rethink, welcomed the decision to better educate the public about the links between cannabis use and mental ill health.

"We want people to have the clearest possible understanding of the link between long-term and early age use of cannabis and schizophrenia," he said.

"This is a huge public health issue with potentially serious consequences for many thousands of the four million regular cannabis users in this country."

Mr Clarke ordered a review of the 2004 reclassification of cannabis last year when he admitted the change had confused the public about the drug's legal status and health risks.

Following the reclassification, a series of health studies warned that cannabis could be linked to schizophrenia and depression.

One, carried out in New Zealand, suggested regular cannabis use increased the risk of developing psychotic symptoms later in life.

Another report, from Maastricht University, concluded that taking the drug "moderately increased" the chance of psychotic symptoms in young people, but added that it had "a much stronger effect in those with evidence of predisposition for psychosis".