Recent years have seen an increase in home-grown cannabis
The strongest type of cannabis - known as skunk - now dominates the UK market, according to new Home Office research.
Skunk now accounts for between 70% and 80% of samples seized by police, compared with 15% six years ago. It is three times stronger than other types.
Senior police officers are urging the government to reclassify cannabis as a Class B drug.
The police are among several bodies giving evidence to the Advisory Council on the Misuse of Drugs.
The Association of Chief Police Officers said reclassification would end confusion over the status of cannabis.
"We do support a re-classification back to B, and there's three reasons for that", Assistant Chief Constable Simon Byrne told BBC News.
"Firstly, we are worried about the rise in the number of cannabis farms we're discovering, which is frankly fuelling a more home-grown market in the more potent type of cannabis.
"That then links in to... our professional concerns about the potency of some of that cannabis, and the effect on some people's mental health."
"And thirdly... I think there's confusion on the streets about whether this drug is legal or not, and that's causing problems for officers who are trying to enforce the law."
Police said the reclassification would help them target organised crime gangs who are profiting from the booming trade in herbal cannabis cultivated in the UK.
The reclassification would be accompanied by stiffer penalties, something that Prime Minister Gordon Brown and Home Secretary Jacqui Smith are believed to support.
But David Blunkett, who downgraded the drug when he was home secretary, said a reversal of his decision would create confusion.
Speaking at the weekend, he said: "Rather than affecting practice on the ground, classifying cannabis back to class B now would simply cause confusion."
And the mental health charity Rethink said cannabis use could be reduced without reclassification, if warnings about the risks were placed on packs of cigarette papers, which are used to roll joints.
Debra Bell, of the Talking About Cannabis Parental Action Group, said: "Skunk cannabis is one of the evils of our time.
"I'm not talking about it just being a gateway drug. It is dangerous in itself."
Skunk is a specific type of cannabis and is so-called because it has a very strong smell, but these days it has become the generic term for stronger forms of the drug.
It is regarded as stronger because it contains much higher levels of the active ingredient in cannabis, tetrahydrocannabinol (THC).
It also contains much lower levels of cannabidiol (CBD), which is an anti-psychotic substance that can moderate the effect of THC on the mental health of users.
A survey for the Advisory Council, by Ipsos/Mori of 1,000 people in England, Wales and Scotland over the age of 16 suggests that most people believe cannabis should be moved into a category reserved for more dangerous drugs.
Of those polled, 32% said cannabis should be moved into Class A - alongside heroin, cocaine and ecstasy; 26% said it should be in Class B and 18% said it should be class C - its currentl category. Eleven per cent said it should be legalised.
But the poll indicates that less than half know that cannabis is currently in class C, and less than a quarter want tougher penalties than those already in force for cannabis possession - the maximum prison term is currently two years.
The mental health charity, Sane, is also giving evidence to the review. Its chief executive, Marjorie Wallace, said there was a significant risk for people who smoke the drug who are under 15 years old.
She said: "Their chances of developing a later illness like schizophrenia can be between two and four times higher - that means there may be about 1,500 people who are developing schizophrenia who, without taking cannabis, might not otherwise have had this long sentence of mental illness."
TYPES OF CANNABIS
Herbal, also known as "grass" or "weed", stronger varieties now dominate the market
Skunk contains three times as much of the active ingredient, THC
Resin, also known as hash, was formerly the most common form of the drug, sold in blocks and crumbled
But the charity Drugscope said it saw no good reason for reclassifying cannabis, so soon after the government decided to make it Class C four years ago.
Harry Shapiro, of Drugscope, told BBC News: "In 2004 it was predicted that there would be a huge increase in the amount of cannabis use as a result of the fact that it was downgraded from a Class B to a C.
"Not only has that not happened, but the government figures suggest that, actually, cannabis use has been falling since then."
The Home Secretary, Jacqui Smith, has said she will maintain an open mind on the subject of re-classification until the Advisory Council on the Misuse of Drugs presents its recommendations.