Gordon Brown is preparing to overrule the advice of the Government’s drug advisory body and upgrade cannabis to a Class B drug, carrying tougher penalties for its possession.

The Advisory Council on the Misuse of Drugs made a recommendation in private that cannabis should remain a Class C drug. Its decision came after the presentation of evidence this week showing a huge increase in the potency of cannabis seized by police but no consistent evidence to support theories that this is causing an increase in schizophrenia.

Of particular concern is the prevalence of skunk, a strain of cannabis which is typically two to three times more powerful than other forms of the drug, although some types can be even stronger. The drug’s potency comes from the high levels of tetrahydrocannabinol (THC) that it contains.

While normal cannabis contains about five per cent of THC, skunk’s concentration is about 10 to 15 per cent.

Ministers have made clear that they are not bound by recommendations from the council and that in the end any decision on classification is a matter of political judgment. It will be only the second time since the council was set up under the Misuse of Drugs Act 1971 to advise ministers that a government has rejected a recommendation on reclassification.

A senior Whitehall official made clear yesterday that Mr Brown was prepared to reclassify cannabis even if the advisory body recommended leaving the drug in the C class. “Whatever the scientific evaluation is, it is the Government’s duty to decide what signal classification sends,” the official said.

The Times disclosed in January that Mr Brown and Jacqui Smith, the Home Secretary, were determined to reverse the decision to downgrade cannabis taken by David Blunkett. Mr Brown asked the committee to carry out a review of the 2004 decision to downgrade cannabis shortly after he became Prime Minister. At the time he appeared to want to distance himself from policies linked with Tony Blair and had already dropped plans for a supercasino.

The council’s decision came after a discussion of research from Keele University on trends in schizophrenia. The study found nothing to support a theory that rising cannabis use over the past three decades had led to increases in the incidence of schizophrenia in later years. Instead, the research by Martin Frisher, of the school of pharmacy, and Ilana Crone, of the academic psychiatry unit, said that between 1996 and 2005 there were significant reductions in the prevalence of schizophrenia and from 2000 onwards in the prevalence of psychoses.

“The data are not consistent with the hypothesis that increasing cannabis use in earlier decades is associated with increasing schizophrenia or psychoses from the mid1990s onwards,” the report said.

Mr Brown signalled his views on cannabis at his press conference this week. He said: “My personal view has been pretty well known for some time.

“Given the changing nature of the stock of cannabis that is coming into the country and greater damage that that appears to be doing to people who use it, there is a stronger case for sending out a signal that cannabis is not only illegal but it is unacceptable.”

The Prime Minister has the backing of senior police officers who have changed their view on reclassification since the original decision.

David Cameron called on the Prime Minister to make a decision on reclassification. He said: “People have had enough of reviews and the Prime Minister should stop dithering and get on and make a decision.”

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