Illicit cannabis factory farmers are arming themselves with sawn-off shotguns, CS sprays and machetes and even setting booby traps to protect their crops from rival gangs, according to a police study published today.

The report for the Association of Chief Police Officers (Acpo) reveals there has been a boom in cannabis production across Britain in the last two years, with nearly 7,000 illegal farms and factories uncovered in 2009/10 alone.

The vast majority are still based in private homes, but the industry has long outgrown its DIY roots of a few plants under the stairs. Large-scale production methods are now the norm in a trade dominated by Chinese and Vietnamese migrants who have been brought into Britain illegally to act as "gardeners".

The police say the impact of the recession means that disused factories, nightclubs and even banks are being used to grow cannabis plants on an industrial scale.

The report, the UK National Problem Profile: Commercial Cultivation of Cannabis, says the largest single cannabis factory ever found was uncovered in July in an industrial unit in Haddenham, Cambridgeshire, where more than 7,600 plants with an estimated value of £2.5m, were seized.

Commander Allan Gibson, of the Metropolitan police, Acpo's lead on cannabis, said that between 2004 and 2007 about 800 cannabis factories a year were being uncovered in Britain. That rose to more than 3,000 a year in 2007/08 and to more than 6,800 in 2009/10, he said.

In the last year almost 750,000 plants had been seized with an estimated value of £85m, compared with 500,000 with an estimated yield of £65m in 2008/09.

Gibson said the level of publicity generated over the reclassification of cannabis in 2008 had led to more people reporting the telltale signs of a hidden factory in their neighbourhood. Police say these indications include blacked-out windows, hot walls, condensation, strange or pungent aromas and people or traffic activity at all hours. These reports had led in turn to an increased focus by law enforcement, including more covert operations and better intelligence-sharing between forces.

The Acpo report discusses the prospect that the scale of homegrown cannabis production is now so great that Britain could become a net exporter of cannabis for the first time in its history.

"Although cannabis continues to be imported into the UK, the domestic commercial cultivation of cannabis has been escalating for a number of years," it says. "The assumption has been made that due to the high frequency at which the UK is producing cannabis, there is now a market for exportation.

"However, there is no intelligence or evidence to suggest this is happening. Data received from the UK Border Agency suggests large amounts of cannabis are still being imported, indicating that the current demand for the drug is so great that domestic production cannot satisfy it."

The study adds that the perception by some criminals that British skunk is of a lower quality than the more powerful Dutch varieties is fuelling this import trade.

The industry now spans the country, with the Met, Greater Manchester, West Midlands and West Yorkshire forces all reporting more than 1,000 factories found in their areas. Glasgow is regarded as a hotspot for drying leaves.

Acpo says that until now cannabis cultivation has been regarded as a "south-east Asian" crime, but the report highlights a growing involvement of white British people in association with Vietnamese and Chinese crime gangs.

The migrants involved have often been trafficked into Britain and subsequently put to work to pay off the £10,000 bill for their journey. Those arrested when police raid a factory are predominantly the "gardeners" left to tend the plants. They are rarely allowed to leave the premises until the crop is harvested.

Disturbingly, the study reports a growing tide of violence in the trade, with an increasing number of reported incidents of cannabis factories being "taxed" by other criminal gangs. This has led those working in the factories to arm themselves to repulse such attacks. Booby traps have also been found, including attempts to electrify window frames, doorknobs and, in one case, a side gate wired up to the mains.

Child trafficking

Children have been identified by the police as having been trafficked from China and Vietnam into Britain specifically to work on cannabis farms. Nearly all are teenagers aged 15 to 17. They are used by the crime gangs to maintain and water the plants, and even break into rival cannabis farms.

"If they are recovered by authorities they are under extreme pressure to abscond from care, with organisers often making threats," says the Acpo study. "A significant number go missing from local authority provided accommodation prior to and after exploitation."

Emotional and physical abuse is often to used ensure their compliance by the gangs. They are extremely wary of talking to the police, perhaps fearful for other family members who have unpaid debts to the traffickers.