Gordon Brown is preparing to override the views of his own expert advisers and tighten the law on cannabis.

Downing Street made it clear yesterday that the Prime Minister is determined to upgrade cannabis from a class C to a class B substance with a maximum jail sentence of five years for possession.

But opposition parties claimed the Government's drugs policy was in chaos after it emerged that Mr Brown would ignore the conclusion of the Advisory Council on the Misuse of Drugs that cannabis should remain a class C substance.

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It will be the first time for 30 years that a prime minister has disregarded the recommendations of the body set up to advise ministers on drugs legislation.

The advisory council was instructed in the summer to review the legislation on cannabis after alarm that super-strong varieties were becoming more popular and that cannabis was linked to schizophrenia.

Mr Brown signalled then that he wanted the controls toughened, but faces embarrassment after the drugs experts and scientists reached the opposite conclusion.

It is understood to have agreed that a link between heavy cannabis use and mental illness could not be proved and to have dismissed suggestions that the downgrading of the drug to a class C substance four years ago sent a confusing message to the public. Only a handful of advisory council members backed restoring cannabis to class B.

Mr Brown's spokesman attempted to play down the decision, insisting the advisory council would not submit its final report until the end of the month. But he confirmed that the Prime Minister did not feel automatically bound by its recommendations: "[Mr Brown] did say he would take advice and look at what they say."

Whitehall sources said Mr Brown was ready to overrule the council. They pointed to his words at a press conference this week when he said he did not want to send any signal that would encourage drug use. One said: "He has a strong view about it." Harriet Harman, the deputy Labour leader, told MPs: "There is new evidence of its dangerousness because a much stronger version of cannabis is being used, with much greater dangers. And there is new evidence of the risk of psychosis caused by cannabis."

The drug was downgraded from class B to class C by David Blunkett, when he was Home Secretary, in January 2004 in a move to enable police to concentrate on tackling hard drugs such as heroin and cocaine.

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Although cannabis use remains a criminal offence, police have in practice tended to confiscate the drug and issue a caution rather than prosecute the users.

Use of the drug among young adults has fallen in the past five years, suggesting downgrading has not encouraged them to experiment. However, "skunk" – said to be two-and-a-half times stronger than traditional cannabis resin – now accounts for about three-quarters of the British market for the drug, compared with 15 per cent in 2002. The advisory council was told at a recent public hearing that users were smoking less of the drug because of its increased potency.

David Cameron, the Conservative leader, who supports the reclassification of cannabis, said: "People have had enough of reviews and the Prime Minister should stop dithering and get on and make a decision."

Chris Huhne, the Liberal Democrat home affairs spokesman, said: "Ministers should stop playing politics with drugs judgments and listen to the advice of the medical experts."

Roger Howard, chief executive of the independent UK Drug Policy Commission, said decisions on drugs legislation should be handed to an independent body that would examine the scientific evidence. He said: "In recent years the process has become muddied by politics."

The Association of Chief Police Officers has called for reclassification, despite having backed the original decision to downgrade.

*The number of drug users and alcoholics claiming incapacity benefits has more than doubled in the past 10 years. More than 100,000 addicts are now claiming a maximum of £87.50 per week. The number claiming the benefit while abusing drugs has risen from 21,900 in 1997 to 49,890 last year. The Tories claimed the payments total about £400m a year.

Classified information

*1979: Advisory Council on the Misuse of Drugs proposes downgrading cannabis to class C.

*1981: Tory government says drugs penalties will never be reduced.

*2000: Blair government agrees that cannabis should be legalised for medical purposes.

*2004: Cannabis is reclassified from a class B to a class C substance, based on evidence from the advisory council.

*2005: The Home Secretary, Charles Clarke, asks the advisory council to re-examine scientific evidence linking cannabis to mental health problems.

*2006: Mr Clarke decides to retain class C status, but calls for further research.

*June 2007: Gordon Brown signals support for upgrading cannabis to class B.