Slaves held in Britain are growing the cannabis that ends up in millions of joints across the UK.
The cultivation of the class B drug by crime gangs in urban homes is being done with the enforced labour of people who often have been trafficked into the country, according to police chiefs.
The Association of Chief Police Officers has launched a week of action against cannabis farms, which it says fuel modern-day slavery.
Chief Superintendent Bill Jephson, the National Police Lead for Cannabis, said: "Cannabis cultivation not only feeds a multimillion-pound black market in increasingly potent and dangerous cannabis, but it is also a key driver in modern slavery, with people forced to work on cannabis farms and strong-armed into servitude, either because of their untraceability as illegal migrants, or because they are in debt to the people in control of the operation."
Official figures show a huge jump in the number of people being trafficked from overseas and put to work on cannabis farms, often inside houses. Between 2011 and 2012, the number rose by 130%. The National Crime Agency found up to eight in 10 of them were children.
Accounts from trafficked workers often involve them being tricked into leaving their home country by the promise of a job in Britain, such as child care. But when they arrive, they fall straight into the hands of criminal gangs that confiscate their documents and put them to work tending to cannabis plants.
Scratch-and-sniff cards that mimic the pungent odour of cannabis were issued to residents by police forces earlier this year. They warned people to look out for tell-tale signs a property was being used as a cannabis farm, such as high heat emissions.
Jephson added: "We are hopeful that the incoming Modern Slavery Bill and continued work by law enforcement partners will aid us in our work in this area."