The government's own scientific advisers last night warned the home secretary, Jacqui Smith, that her decision to upgrade the legal status of cannabis would not work in curbing its widespread abuse.

Professor Sir Michael Rawlins, chairman of the Advisory Council on the Misuse of Drugs (ACMD), said moving cannabis from class C to class B "is neither warranted, nor will it achieve its desired effect".

Smith told the House of Commons yesterday that she had to take into consideration public perceptions and the pressures on policing as well as the advice of the advisory council.

"There is a compelling case to act now rather than risk the future health of young people," she said. "Where there is a clear and serious problem, but doubt about the harm that will be caused, we must err on the side of caution and protect the public. I make no apology for that I am not prepared to 'wait and see'."

The ACMD, the government's own expert body on drugs, decided by 20 votes to three to recommend that cannabis remain a class C drug. Its nine-month review concluded that while more potent, homegrown strains of herbal cannabis, such as skunk, now dominate the British market, the evidence of a substantial link with mental illness remains weak.

Rawlins said that the government had the right to consider other factors but warned that reclassification would make little difference to the levels of consumption: "We know that for people who smoke cannabis, it makes no difference to them whether it is class A, B or C. What is important is a really vigorous public health response."

The home secretary told MPs she had accepted the 20 other recommendations made by the ACMD, including a tougher enforcement campaign against cannabis farms, a crackdown on "head shops" which sell cannabis paraphernalia, including seeds, and a renewed public health campaign.

Smith confirmed that the police approach to those under 18 found in possession of small amounts would remain unchanged. The police will continue to have the discretion to issue warnings to most adults found with small amounts of dope.

But the home secretary made it clear that she has asked the police to propose a tougher enforcement policy with escalating penalties for repeat offenders including fixed penalty fines and in cases involving aggravating factors, such as those which happen in a psychiatric hospital, a prison or a school.

Supplying the drug to children, students, psychiatric patients or prisoners will also attract harsher sentences.

DrugScope, the leading drugs information charity, said in a statement that it was disappointed the government had ignored the ACMD's advice: "Unfortunately, the message given by this decision is that drugs policy can be driven as much by political considerations, media headlines and scare stories as by the evidence."

Professor Colin Blakemore, the former head of the Medical Research Council, said that cannabis use had fallen since 2004 and restoring the drug to class B status would be unlikely to protect those people who were most vulnerable, but would increase their chance of getting a criminal record. "The confusion over cannabis highlights the need for a proper overhaul of the present classification system, which the public neither understands or respects," Blakemore said.

The shadow home secretary, David Davis, welcomed the reclassification decision, but asked why it had taken so long.

"The government's lax approach to drugs is the hallmark of our broken society under Labour," he told MPs.

"The UK has the worst level of overall drug abuse in Europe. Drug crimes have increased by almost a half under this government."

Cannabis was downgraded from class B under Tony Blair, but Gordon Brown announced a review of its status soon after becoming prime minister.

The Liberal Democrats' home affairs spokesman, Chris Huhne, said it was "crazy" to ignore the government's scientific advisers by raising the maximum prison sentence for cannabis possession to five years. "Cannabis use is falling, as is the incidence of psychosis," he said. "We need public education, not public flagellation."