More than twice as many secret cannabis factories were found in past year as four years ago, chief constables' report reveals

Clandestine cannabis factories are booming in Britain, with the police detecting more than 7,800 in the past year – more than double the number found four years ago.

Chief constables say the rise in illicit cannabis production is being fuelled by the increasing involvement of organised crime groups, who see it as a "low-risk and highly profitable criminal business".

But the police also acknowledge that the recession and pressure on household budgets has led to a rise in a "grow your own" trade, with intelligence reports showing an increase in sales of seeds and equipment from local "head shops" to customers wanting to grow a few plants at home for their own use.

The third "problem profile" of UK commercial cultivation of cannabis, published on Monday by the Association of Chief Police Officers (Acpo), says police have seized more than 1.1m plants with an estimated street value of £207m in the past two years.

The survey, published every two years, confirms a majority of the cannabis used in Britain is now home-grown rather than imported, with some claiming more than 80% is intensively cultivated domestic herbal cannabis.

But police chiefs say there is a shift away from large-scale cannabis factories in disused industrial and commercial buildings such as empty cinemas, shops and banks back to smaller houses and flats, often on suburban streets. The organised crime gangs behind the trade in home-grown cannabis are reacting to police crackdowns by moving away from large-scale premises to employing a large number of "gardeners" to operate small-scale "grow sites" or factories across several residential areas. "This spreads the risk and minimises the potential for detection and financial loss," says the report.

It acknowledges there has been a proliferation of users claiming to be involved in growing plants at home for personal use. It says the economic downturn and a fall in the weight of the average street cannabis deal have been accompanied by a rise in the amount of home-grown cultivation for personal use.

This has also led to a rise in the amount of home-grown cannabis being supplied to friends and acquaintances through "social dealing".

But the report says in many cases the number of plants seized in raids is well above 25, which is regarded as the legal minimum to be prosecuted for commercial cultivation and for which the indicative minimum sentence is two to five years.

The police say they assume anyone who grows more than 10 plants is likely to have a surplus and therefore to be supplying others.

Police say 7,865 cannabis farms or factories were detected in Britain in 2011-12, compared with 6,866 in 2009-10 – a 15% rise since the last time the UK problem profile was published. Just over 3,000 were detected in 2007-08.

Police chiefs estimate the crime figures for the year to March 2012 will show 16,464 offences recorded for commercial cannabis production compared with 14,982 for 2010-2011.

The report plays down previous claims that so much cannabis is being grown in Britain that it has become a net exporter. "Intelligence indicates that UK organised crime gangs may supply drugs to the continent to fill a gap in the market but there is no evidence of widespread export," it says.

The Acpo report admits that tackling cannabis factories is not considered a priority for most British police forces, with operations to tackle the supply of class A drugs such as heroin and cocaine a higher priority. The police also see the dismantling of cannabis factories as a short-term solution which misses opportunities for further investigation into other potentially linked factories.

Allan Gibson, of Acpo, said: "Commercial cannabis cultivation continues to pose a significant risk to the UK. Increasing numbers of organised crime groups are diverting into this area of criminality but we are determined to continue to disrupt such networks and reduce the harm caused by drugs.

"This profile provides a detailed analysis of the current threat from commercial cultivation of cannabis and the work undertaken by law enforcement agencies to combat the threat. It provides a framework to facilitate future planning and decision-making for preventative, legislative and enforcement activity to make the UK a hostile environment for cannabis cultivators."

• This article was amended on 30 April 2012. The original said 1.9m cannabis plants had been seized in the past two years. Acpo has since corrected its figures to 1.1m.