Hundreds of children are being trafficked from Vietnam and forced to work in cannabis factories across Britain by criminal gangs.
Campaigners say that almost one child a week is found working in appalling conditions in houses that have been converted to cultivate cannabis.
Children as young as 11 are brought to the country on the promise of a better life by criminal gangs and are then held captive and threatened by criminal gangs.
Those who are freed after police raids are often treated as criminals and are sent to young offenders centres, and many more simply fall through the cracks and are never heard of again.
After dominating the illegal cannabis trade in Canada, Vietnamese gangs have filtered over to Britain since the drug was given a lighter classification in 2004.
More than 1,500 cannabis farms have been closed down in London alone over the last two years – three times the amount in 2003-5.
Gangs can earn up to £300,000 profit a year from a three-bedroom house converted into a cannabis factory.
Young Vietnamese trapped into working in factories can be stopped from running away by electric wire over windows and doors or threats of violence.
"These young people may have thought that they were coming to the country for a new life, but then they find themselves in a situation where they are being forced to work in cannabis factories to pay back debts to criminal bosses," said Christine Beddoe, director of campaign group End Child Prostitution, Child Pornography and the Trafficking of Children for Sexual Purposes (ECPAT).
"The gang bosses keep them there out of fear by making threats not only to themselves but also to their families in Vietnam."
In September 2006, London's Metropolitan police set up Operation Keymer, a community initiative to aide the detection of houses that had been converted for cannabis production.
Most set-ups follow the traditional blue-print of over-head lights, fans and pot plants.
It typically costs around £30,000 for criminal gangs to set up a 'factory' but they often only last around six months before they are discovered.
In that time gangs are often compelled to produce as much cannabis as possible to ensure that the profits are worth the outlay.
Many will tap into mains electricity power in order to power high voltage lamps and fans. In one property police found that equipment had been wired up to nearby street lamps.
Properties can be detected a number of ways, from 'community intelligence' – where residents report suspicious activity at a property, to energy companies tipping off police about properties which are found to be bypassing the mains electricity.
An increasing way in which police are detecting properties is through heat sensors attached to helicopters.
Plants are found everywhere in the property, from baths to the toilet, and walls may be lined with cellophane or foil to reflect heat back into the property.
Under the effect of the heat, the plants produce a powerful odour, which is strong enough that police are advised to wear masks upon entering properties.
Amidst all of this, child slaves, or 'ghosts' are forced to work, watering the plants and sleeping on mattresses.
A spokesman for charity DrugScope said: "It's a dangerous environment for people to be living in. There is always the potential for disaster."
Despite this, once they are found by police they are often arrested and treated as criminals.
Ms Beddoe said: "Even though they are victims of human trafficking they are still seen as part of criminal organisations. They are being prosecuted for drug offences and are getting sentenced into young offenders institutes.
"Many of them go missing because they are under threat from gangs and then go missing and many of them are never found. This is adding to the cycle of abuse and exploitation."
In August, young Vietnamese boy Tuan Nguyen, was taken into the care of Salford City Council after being found guarding a cannabis factory raided by Greater Manchester Police.
However shortly afterwards he went missing and was feared to have been kidnapped back by the criminal gang who brought him here.
However he was found at a property in Oxfordshire on October 21, although police would not say where he was found, they were able to confirm he was not working at a cannabis factory at the time.
Paul Woltman, assistant director of children's services at Salford City Council said: "Our understanding from this lad, from information we have gleaned from him, is that he was abandoned by his parents, then brought up by an older woman, he called grandmother. She has died and he has become a street boy. Then through a series of events he has ended up here."
Det Insp Neil Hutchison, of the Met's Serious and Organised Crime Squad, who was also central to the planning of Operation Keymer, told magazine Drugslink: "Some of the Vietnamese growers have been smuggled over here and have paid good money.
"But once here they find themselves under the thumb of gangs who will push them into working on cannabis farms. There is an element of compulsion.
Some will get paid a pittance, while others get a percentage of the profits – it differs between gangs."
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