Anyone caught smoking cannabis could now get five years in jail after the government increased the punishment for getting hold of the drug.
Gordon Brown has pushed through plans to return cannabis to its class B status, five years after Tony Blair downgraded it to class C.
The decision was made against the advice of the government's own panel of independent drug experts.
The Home Secretary Jacqui Smith told Newsbeat there is growing evidence that stronger strains of skunk cannabis can cause severe mental health problems like schizophrenia.
"The idea that cannabis is some sort of gentle drug that doesn't have any impact on the people who use it themselves or on those supplying it isn't born out by the evidence," she said.
"The sort of cannabis available on the streets is significantly stronger even over the last two or three years than it has been previously."
Teenage cannabis users explain why they smoke the drug
Punishment for over-18s caught with the drug will increase from a typical "confiscate and warning" to a possible penalty notice and fine followed by arrest and prosecution for repeat offences.
But campaigners say the change will do nothing to reduce the amount of people taking the drug and cannabis users have told Newsbeat they are angry at the decision.
Jenny from Glasgow said: "I think it should be legal. You do a lot worse things on alcohol. You go out, get bladdered and you're sick everywhere. We are just sat at home giving a giggle. We don't think we are doing anything wrong."
The 'class' system divides illegal drugs into different categories - A, B or C - depending on how harmful they are thought to be.
The most dangerous drugs like heroin and crack cocaine are in class A with the least harmful like tranquillisers and some prescription-only painkillers in class C.
Class A: Heroin, cocaine, LSD, crack, ecstasy, magic mushrooms, some amphetamines (injected)
Class B: Cannabis, amphetamines (speed), some painkillers
Class C: Ketamine, GHB, tranquillisers, some painkillers
In 2004 the government took advice from a group of independent drug experts and shifted cannabis down from a class B to a class C substance.
It meant the maximum punishment for possession was reduced from five years in jail to two years.
In reality it was normally treated as a non-arrestable offence.
Anyone caught with a small amount of cannabis could expect to get it confiscated and let off with a warning.
When Gordon Brown took over as Prime Minister he asked the same independent panel of experts to review its recommendation.
After reporting back last year, they said cannabis should remain a class C drug.
Professor David Nutt, who chairs the government's Advisory Council on the Misuse of Drugs, told Newsbeat: "It is pretty clear that cannabis should be class C because it doesn't cause a great deal of harm to society.
Jacqui Smith: Cannabis 'no gentle drug'
"Some individuals do get unpleasant mental reactions but they are relatively small in number."
He said there is currently no evidence that higher strength types of skunk cannabis are any more dangerous than weaker varieties, although more research is needed in this area.
Instead users may take less of the drug in the same way that someone who drinks alcohol will consume less whisky than beer.
"We are not convinced that moving it to class B with the possibility of five years imprisonment for possession will have any beneficial effects," he said.
"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."
Last year, parliament voted to reject the findings of the advisory council and reclassify the drug as class B. The decision was supported by the Association of Chief Police Officers and a number of mental health charities concerned about the effects of higher strength skunk.
Class A: Maximum seven years imprisonment and unlimited fine; life imprisonment for supply
Class B: Maximum five years imprisonment and £5,000 fine; 14 years for supply
Class C: Maximum two years imprisonment and £2,500; 14 years for supply
Detective Chief Inspector Andy West from Lincolnshire told Newsbeat: "In practice it 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."
Under a new 'three-strikes' regime in England and Wales, cannabis users over 18 are still expected to get a warning for possession on their first offence.
If caught for a second time they will get a penalty notice and a £80 fine.
People are far more prone now to take cannabis, alcohol and other substances and get completely paralytic drunk and bombed out of their minds
Detective Chief Inspector Andy West
A third offence - or strike - will mean arrest followed by either release without charge, a caution or prosecution with possible jail time.
There will be no change in the law for under-18s with police likely to either issue a warning or a caution depending on the circumstances.
In Scotland and Northern Ireland the system is different.
Anyone found with cannabis will continue to be reported to the Procurator Fiscal of the Public Prosecution Service where a decision to prosecute or issue a caution will be made.
In reality drug charities have told Newsbeat that police may still decide to issue an informal warning depending on the severity of the offence.
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