Children are being trafficked into the UK to work in so-called 'cannabis factories' according to police.
The factories grow cannabis on an industrial scale.
They are typically located in private houses, but sometimes in agricultural and commercial properties.
A study for the Association of Chief Police Officers has found children are being used to tend crops, to illegally divert electricity for the factories, and to break into rival sites.
Even if the children are found they rarely talk about what has happened to them and often run away from local authority care.
The report into the commercial cultivation of cannabis shows how the number of factories discovered by the police has grown massively in recent years.
Between 2004 and 2007 police discovered an average of 800 factories a year.
In 2007/08 that rose to just over 3,000, and by 2009/10 it ballooned to almost 7,000.
ACPO says that rise is being driven by a number of factors. Commander Allan Gibson is the Association's lead on tackling cannabis production.
"The police response is now stronger and more effective through better co-ordination and intelligence sharing between forces and other agencies and more covert operations against the operators.
The level of publicity around cannabis since its reclassification in 2008 has meant that more members of the community are now reporting any unusual signs of habitation in buildings and houses which is leading to more detections."
Criminals have traditionally used residential homes for the factories, blacking out windows and installing high intensity lighting and a watering system to propagate the plants.
'Gardeners' rarely let out
The report explains how illegal Chinese and Vietnamese immigrants are trafficked into the country and then put to work as 'gardeners' in the factories.
They can pay up to £10,000 per person to be smuggled into the UK and many then have to pay off their debts by tending to the cannabis plants.
They are often left instructions in their own language telling them how and when to feed the plants.
They are rarely, if ever, allowed to leave the property until the crop is ready to be harvested, with food and other necessities being brought to them.
The report says children have been trafficked into the UK to work in factories, to divert electricity and to raid rival cannabis farms.
Even if they are found by the authorities getting their co-operation has proved extremely difficult.
"Children identified as victims of trafficking appear extremely wary of authorities and communicate little about their experiences in exploitation or about their captors.
"This may be because they are fearful for family members, as many will have unpaid debts, perhaps explaining their reasons for not disclosing information and going missing from care."
As well as growing cannabis the intelligence suggests the criminals who grow cannabis are involved in a range of criminal activity.
That includes producing and distributing most types and classes of drugs, counterfeit currency, fake DVDs, money laundering, prostitution, forgery and what they call 'bad on bad' crimes such as extortion, blackmail, kidnap, robbery and assault.