The government today defied the advice of drug experts and upgraded cannabis from class C to class B.
The home secretary, Jacqui Smith, announced a new system of escalating penalties for adults caught in possession of small amounts of the drug from early next year.
Tougher sanctions will replace the current system of police warnings, and officers will be able to arrest first-time offenders.
Smith justified her decision by highlighting the strength of "skunk" strains of herbal cannabis now widely available.
Last week, Gordon Brown warned of the "more lethal quality" of much of the cannabis now available, described it as a gateway drug, and said that the reclassification was needed to "send a message to young people that it was unacceptable".
The home secretary told the Commons today: "Reclassification reflects the fact that skunk, a much stronger type of the drug, now dominates the cannabis market."
She said it accounted for 81% of cannabis available on the streets compared to just 30% in 2002.
The average age of first use is 13 years old and young people may binge on skunk in the same way as alcohol, trying to achieve the maximum effect, Smith told MPs, saying that the Advisory Council on the Misuse of Drugs had found that the consequences of this binge smoking "may be serious to their [users'] mental health".
Smith said that the reclassification would mean "more robust" enforcement of laws banning the supply and possession of cannabis and a new approach to tackling cannabis farms and the organised criminals behind them.
She also announced that she would work with the Association of Chief Police Officers (Acpo) to find ways of using existing laws to curtail the trade in cannabis paraphernalia such as pipes.
Smith's decision to upgrade the drug went against the recommendations of the government's scientific experts, the Advisory Council on the Misuse of Drugs, which was asked by Smith to take its third look at cannabis classification in recent years.
The council's advice, published today, was that cannabis should remain class C.
The ACMD chair, Professor Sir Michael Rawlins, refused to criticise the home secretary, saying that the ACMD's recommendations were based entirely on harmfulness - but that the government had the right to consider other factors.
He said: "We don't take into account 'the message', we don't take into account policing priorities; we are obliged by law only to take into account the harmfulness.
"The government may want to take into account other matters. That's their right; they are the government. We are only an advisory committee and from time to time governments, for their own reasons, may wish to ignore the advice."
He insisted that the recommendation on classification, ignored by the home secretary, was the least important of the 21 in the report.
"The most important ones are the ones related to public health. The thing I take great comfort from is that all the other recommendations are fully accepted and they are the ones that I think will make a difference."
He said he would be surprised if there were any resignations from the council, and advised members to "tough it out".
His council heard evidence that the potency of homegrown herbal cannabis tended to be two and a half times that of imported resin. But they said users now often moderated their intake.
They were also told that the incidence of new schizophrenia cases reported to GPs had gone down, not up, between 1998 and 2005, indicating a weak link between increased potency and use in the past two decades and mental health problems.
Since cannabis was downgraded in 2004 the proportion of young people using it has fallen each year from 25.3% in 2003-04 to 20.9% now. Among those aged 16 to 59, the proportion over the same period has fallen from 10.8% to 8.2%, according to the British Crime Survey.
The shadow home secretary, David Davis, said that he supported the decision to upgrade the drug but he criticised Labour's indecision over its classification.
He said: "The government's lax approach to drugs is the hallmark of our broken society under Labour."
"This long-awaited U-turn has followed delay, dithering and indecision when the country cries out for leadership," he added.
Davis accused the prime minister of wasting a year by announcing his policy intentions and then handing it to an advisory committee which he finally ignored.
Chris Huhne, the Liberal Democrats' home affairs spokesman, said: "The crackdown on skunk farms makes sense, but it is crazy to ignore the Advisory Council on the Misuse of Drugs by raising the maximum prison sentence for cannabis possession to five years.
"Cannabis use is falling, as is the incidence of psychosis. We need public
education, not public flagellation."
Campaigners for drug law reform, who questioned the relevance of the drug classification system, which dates back to 1971, showed little surprise at today's announcement.
Ben Lynam of the UK Drug Policy Commission, an independent group, said that he was concerned that the home secretary had chosen to ignore the recommendations of a commission she had instigated.
"We are very concerned about how political this debate has become. The drug classification system is a very poor vehicle for communicating the risk of drug use to potential users.
"It needs to be taken out of the political arena by using an independent body like the ACMD but that is listened to and gives recommendations based on scientific evidence rather than political expediency."
Danny Kushlick of Transform, another drug law reform lobby group, said: "This argument over drug classification is distracting attention away from where the real and substantive debate on whether drugs should be illegal or regulated."