The government's decision to ignore scientific advice has been controversial
The reclassification of cannabis from a Class C to a Class B drug means a higher maximum jail sentence for possession.
Announcing the move in May 2008, then 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 said.
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".
There are three sub-species of the cannabis plant from which the drug can be derived - sativa, indica and ruderalis.
All three grow indigenously in Asia and each is said to produce a different kind of "high".
The plant has been used in various forms as an intoxicant since Assyrian times, but today it is usually produced in herbal form ("grass" or "weed") or as a resin ("dope").
Cannabis and the law
Possession of Class B Drugs (including cannabis and amphetamines) carries a maximum sentence of five years in jail
Possession of Class C drugs (including ketamine and GHB) carries a maximum sentence of two years in jail
Supply of Class B and C drugs can attract a maximum sentence of 14 years in jail
Cannabis 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).
In England and Wales, a first offence of possession of cannabis will usually attract a warning
With "aggravating factors", such as smoking in the presence of children or public order offences, arrest is likely
An £80 spot fine (penalty notice of disorder - PND) is proposed for a second offence
A third offence will usually mean arrest and a court appearance
In Scotland, all offences have to be reported to the Procurator Fiscal
Possession of small amounts of cannabis is usually dealt by a Fiscal Fine (not included on someone's criminal record) or no action being taken
No formal guidelines apply in Northern Ireland
PNDs are not introduced there, but in practice police procedure closely follows England and Wales
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 home secretary 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.
Jacqui Smith explains her stance to Radio One Newsbeat listeners
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".
The BBC News website and Radio One's Newsbeat spoke to people about the reclassification:
PROFESSOR DAVID NUTT, ADVISORY COUNCIL ON THE MISUSE OF DRUGS
"Some individuals do get unpleasant mental reactions but they are relatively small in number.
"We are not convinced that moving it to Class B with the possibility of five years' imprisonment for possession will have any beneficial effects.
"There are potential risks of criminalising people who are using a drug which doesn't harm other people, only themselves.
"Alcohol is more harmful [than cannabis] - to the population, certainly, and to the individual, possibly."
MARJORIE WALLACE, MENTAL HEALTH CHARITY SANE
"We support the proposal to reclassify it as a Class B drug.
"Cannabis, especially in its more toxic varieties, can double the chance of developing severe mental illness in a significant minority of people, particularly the young, whose brains are still developing.
"While we do not yet know the cause of psychotic illness, or the ways in which drugs such as cannabis may trigger breakdown, relapse, and worsen outcomes, we need to maintain a clear message that it is dangerous to the 10-20% of people who may be at risk but do not know it."
DANNY KUSHLICK, DRUGS POLICY THINK TANK TRANSFORM
"Nobody is going to be put off smoking cannabis by the decision to reclassify it.
"It's a decision that has been taken for political reasons, to trump the Tories' law and order agenda, rather than for any scientific reason.
"Our position is - if cannabis can be dangerous to a few, yet two million people regularly smoke it - we should have a regulated and supervised market for it, rather than putting its distribution in the hands of criminals."
DET CH SUPT ANDY WEST, LINCOLNSHIRE POLICE
Det Ch Supt West was in charge of the investigation into the murder of Stevie Barton, a student who was killed by paranoid cannabis user Marc Middlebrook in 2007.
"[Reclassification] will send out a message.
"People are far more prone now to take cannabis, alcohol and other substances and get completely paralytic drunk and bombed out of their minds.
"It's not right. We've all got to take some responsibility for our actions. That decision [to downgrade cannabis in 2004], in my mind, made it a lot easier for people to excuse their own bad behaviour."
MARTIN BARNES, INDEPENDENT DRUGS CHARITY DRUGSCOPE
"We remain concerned that the government rejected independent expert advice on cannabis classification - a worrying precedent has been set.
"There is no evidence that moving the drug to Class B will of itself reduce levels of use, harm or availability nor is there evidence that the public wants to see tougher penalties for cannabis possession, particularly for young people.
"Cannabis is a harmful drug and we do need to be vigilant. Using the classification system to 'send out a message' is a blunt and questionable approach, particularly if it risks undermining... credibility."
STATEMENT FROM MAGISTRATES' ASSOCIATION
"The singling-out of possession of cannabis for disposal through a PND [penalty notice of disorder] sets it aside from the other Class B drugs and sends an unacceptable signal to those who use it.
"The significant campaign by the Magistrates' Association and other agencies to reclassify the drug will be undermined if this is implemented."
BOB, CANNABIS USER
Bob is a 45-year-old IT worker who lives in a town on the Kent coast and has been using cannabis for 26 years. He says he smokes "five to 25" joints a day.
"What I'd like to ask politicians is what harm am I doing by staying in, rolling a few spliffs [joints] and watching television?
"Maybe I should be out down the pub, getting pissed, getting in punch-ups and shouting my head off? That seems the establishment's preferred option.
"The only harm I do is to my own lungs. The fact that I'm considered a criminal is, I think, a fundamental breach of my human rights."
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