Cannabis has been reclassified by the government from a Class C to a Class B drug, carrying a higher maximum jail sentence for possession.
Home Secretary Jacqui Smith said there was "uncertainty at the least" on the future impact on young people's mental health as a result of using cannabis.
Therefore she was going to "err on the side of caution and protect the public" by upping the classification level, she went on.
Cannabis users in the UK tend to smoke the drug in "joints" although it can also be eaten in "hash cakes" or drunk (the active ingredient is soluble in milk) in "hash coffee".
The drug, derived from the flower buds of the cannabis sativa plant, grows indigenously in Asia and has been used as an intoxicant since Assyrian times.
In modern times, it is usually produced in herbal form ("grass" or "weed") or as a resin ("dope").
Its use in Britain was very limited until the 1950s, when it was brought to the country in slightly larger quantities by West Indian immigrants.
The plant had been introduced to the new world by plantation workers from the Indian subcontinent.
The hippy sub-culture of the mid to late 60s spread the drug's use.
Cannabis has been illegal in Britain since 1928, when a law derived from the International Convention on Narcotics Control came into effect.
The convention was mainly concerned with controlling the Far East's opium problem, but cannabis was also included by the League of Nations in its list of substances which member states should seek to control, at the behest of Egyptian and Turkish delegates.
The law was applied in a rather erratic manner throughout the UK until the Misuse of Drugs Act in 1971, which introduced the drug classification system and sentencing guidelines.
In 2004, the then Home Secretary David Blunkett approved the reclassification of cannabis from Class B (which it had been since the 1971 act) to Class C.
The move followed a report from the Home Affairs Select Committee and advice from an independent panel of experts - the Advisory Council on the Misuse of Drugs (ACMD).
Mr Blunkett said he hoped the move would free the police to concentrate on more "serious drugs". Advice to forces in England and Wales said possession of the drug in small quantities would normally be dealt with by an informal warning or caution.
In May 2008, however, the current Home Secretary Jacqui Smith announced that she would reverse the 2004 decision and put cannabis back into category B.
The move went against the ACMD's latest recommendations, but was, she said, necessary because of research linking heavy use of the drug with schizophrenia and other mental illnesses.
Ms Smith said she was particularly concerned over the rise in consumption of super-strength strains of cannabis, such as "skunk". The Home Office say such strains account for 80% of all cannabis seizures in the UK.
There are plans to introduce a "three strikes" system for cannabis possession, where a warning will be followed by an £80 spot fine (called a "penalty notice of disorder") and a third offence will render a person liable to arrest and prosecution.
The spot fine proposal, as part of a list of 21 more offences for which police could impose penalties, is due for further consultation after magistrates expressed concerns about taking offences away from the courts system.
But the government says it wants spot fines for cannabis possession "as soon as possible".
-- BBC News