· Fall in users since Blunkett declassified drug in 2004 · Study finds no evidence of link with schizophrenia

Gordon Brown intends to tighten the penalties for cannabis possession despite a clear recommendation by the government's own drug experts that it should remain a class C drug.

The Advisory Council on the Misuse of Drugs took its decision in private on Tuesday after presentations of research confirming the increased potency of most cannabis available on the street, but dispelling fears the drug has led to a growth in mental health problems.

Downing Street made it clear last night that the prime minister will press ahead with the reclassification of cannabis to a class B drug, with a maximum five-year prison sentence for possession, whatever the scientific advice from the experts when they deliver their report on April 28. Chief constables said they would back a decision to change the law.

Experts who attended the private meeting have confirmed to the Guardian that they have decided to recommend it remains a class C drug.

The advisory council was asked by the home secretary, Jacqui Smith, last July to look again at the legal classification of cannabis because of public concern over the impact on mental health of stronger forms of the drug known as skunk. She acknowledged cannabis use had fallen significantly since the decision was taken in 2004 by David Blunkett to relax the penalties on the drug by downgrading it.

The Guardian has learned that a Home Office research paper presented to the meeting on Tuesday confirms that a more potent homegrown herbal cannabis has overtaken imported Moroccan resin and now accounts for 85% of seizures by police from individuals on the street.

The Guardian understands that at the ACMD meeting, the 23 medical and drug experts heard a personal presentation on the possible mental health impacts of higher potency cannabis from psychologist Dr Martin Frisher, of Keele University pharmacy school. The presentation used unpublished data from a confidential report he has drawn up for the Home Office.

He and his colleague Professor Illana Crome, of Keele's academic psychiatry unit, used data from 183 GP practices across Britain between 1996 and 2005 to work out whether schizophrenia is on the rise, and whether it can be linked to the increase in cannabis use since the 1970s.

Their confidential paper found that between 1996 and 2005 there had been significant reductions in the incidence and prevalence of schizophrenia. From 2000 onwards there were also significant reductions in the prevalence of psychoses.

The authors say this data is "not consistent with the hypothesis that increasing cannabis use in earlier decades is associated with increasing schizophrenia or psychoses from the mid-1990s onwards."

The research was designed to test the predictions of a previous study, published last year by Dr Matthew Hickman and colleagues at Bristol University, which suggested that changes in cannabis use since the 1970s would lead to expectations that the prevalence of schizophrenia cases would increase by 10% to 20% over the next few years. The results show that not only has there been no increase but the number of new cases has dropped.

Scientists stress that the vexed question of whether cannabis causes schizophrenia remains unclear. They currently think that 8% to 10% of patients who suffer schizophrenia-like illnesses would not have had the condition had they not used cannabis. These figures are debated and the role of genetic factors is still poorly understood.

Last night the Prime Minister's spokesman said the ACMD decision appeared to be based on a single presentation, and the committee had yet to reach any final conclusions. But he confirmed that the prime minister stood by his comments before the meeting on Tuesday that the government needed to send out a signal that cannabis use was not just illegal but also unacceptable.

Backstory

1970 Special class B category, halfway between "hard" and "soft" drugs, created for cannabis as a compromise between Labour home secretary James Callaghan, who believed it was as dangerous as heroin, and a "student faction" in cabinet who did not.

1978 Advisory council on misuse of drugs (ACMD) recommends downgrading cannabis from class B to class C to put it on a par with tranquillisers. Labour home secretary Merlyn Rees rejects advice.

2002 ACMD looks again at legal status of cannabis at request of home secretary David Blunkett, and again recommends downgrading it from B to C, saying it was less harmful than other class B drugs such as amphetamines.

2004 Blunkett downgrades cannabis. It is still illegal but police adopt a policy of "confiscate and warn". Maximum prison sentence for possession cut from five to two years.

2005 Charles Clarke asks ACMD to review evidence on effects of strong cannabis on mental health. The council decides to confirms its status as a class C drug but issues a reminder of its harmfulness.

2007 Home secretary Jacqui Smith requests further review amid anxiety about more potent strains and impact on mental health.