The home secretary has said she has an open mind about cannabis as the prime minister's promised review of cannabis reclassification gathers pace.
The Advisory Council on the Misuse of Drugs has begun hearing evidence on claims cannabis should be reclassified as a class B drug, amid apparent evidence of fresh harm caused by the drug.
Senior police officers today told the council they were "unequivocally" in favour of reclassification.
Simon Byrne, assistant chief constable of the Association of Chief Police Officers (ACPO) said reclassification would end confusion about the legal status of cannabis.
When cannabis was downgraded to a class C drug in 2004, supporters of the move claimed it would allow police forces to concentrate on harder drugs.
But Mr Bryne said reclassification had sparked a rise in cannabis farms, with criminal gangs now thought to be fuelling supply.
He told Radio 4's Today programme: "The bottom line from a police point of view is that since reclassification four years ago we have seen a significant rise in cannabis farms, which point to increasing use of organised crime in this particular market; that's a worry for us."
The Conservatives called on the government to take immediate action to reclassify cannabis in the light of Mr Byrne's comments.
Shadow home secretary David Davis said: "The government should stop dithering and take immediate action to reclassify cannabis as a class B drug."
He argues cannabis is a gateway drug, encouraging the use of harder drugs, and a major cause of crime.
"It has real and tragic consequences for the mental health of so many people," Mr Davis argued.
Critics claim those arguing for cannabis to remain a Class C drug have a benign view of the substance and have not been exposed to the stronger strains common today.
Home Office research suggests the market is now dominated by the stronger "skunk" variant.
It made up 70 to 80 per cent of samples seized by police last year, compared to 15 per cent of samples six years ago.
Amid increasing concerns about the effects of widespread cannabis use, Mr Brown announced a review of the drug's classification shortly after becoming prime minister last year.