The government today defied the advice of drug experts and upgraded cannabis from class C to class B.
Tougher sanctions will replace the current system of police warnings, and officers will be able to arrest first-time offenders.
Jacqui Smith, home secretary, 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".
Smith 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.