More than one million cannabis plants have been seized in the UK over the past two years as the number of cannabis farms discovered has doubled from four years ago.

Police officers have confiscated a total of 1,096,797 cannabis plants over a two-year period at a street value of £207m, according to a report from the Association of Chief Police Officers (Acpo).

Police intelligence has said the increase in the number of cannabis farms poses a "significant risk" to the UK, because the rising commercial cultivation of cannabis leads to an increase in burglary, violence and the use of firearms.

It is now thought that more people are growing cannabis for personal use to avoid giving money to gangs who control the drugs.

Figures show that a total of 7,865 cannabis farms were discovered in 2011/12, an increase on the 4,951 that were identified in 2008/09 and more than double the 3,032 farms found in 2007/08.

The report suggests people are setting up smaller farms in residential properties to decrease the risk of detection. It also acknowledges that the recession has led to an increase in the number of people growing cannabis for their own use, as demonstrated by the rise in the number of offences for personal cultivation.

The number of offences related to cannabis production is also increasing, up from 14,982 in 2010/11 to 16,464 in 2011/12.

The area with the most number of cannabis farms discovered in the UK was West Yorkshire with 936, equivalent to 42 factories per 100,000 people, according to Acpo's figures.

South Yorkshire, where 851 farms were discovered, had 64 farms per 100,000 people, the highest in the country.

Peter Reynolds, elected leader of cannabis reform group Clear, believes the rise of cannabis farms for personal use may be attributable to people preferring to grow their own drugs than give money to gangs.

"There is a very strong, continuous demand for cannabis, which is not going to go away," he said.

"People are more educated about cannabis now and more people are educated about its risks and aware that it is 100 times safer than alcohol.

"People who have made the conscious decision to use cannabis are now doing so without passing on the money to illegal gangs, who use the money for activities such as human trafficking."

Commander Allan Gibson of the Metropolitan Police, Acpo's lead on cannabis cultivation, 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."

Danny Kushlick, head of external affairs for the Transform Drug Policy Foundation, said believes taxing and regulating cannabis will help stop the problems highlighted in Acpo's report .

"The lucrative skunk business is just one more way to make untaxed profits for those who are prepared to take the risk," Kushlick said.

"Drug law enforcement is counterproductive at any time, but in an era of financial stricture it is tantamount to burning money.

"Cannabis should be legally regulated precisely because of these risks, not because it is perceived as a safe drug. It would enable the government to take over the trade from organised crime, to licence growers and vendors, control price and purity, and to put in place age controls on purchasing."

According to the 2010/11 British Crime Survey, an estimated 6.8 percent of 16 to 59-year-olds (2.2 million people) had used cannabis in the last year, an increase from 6.6 percent in 2009/10.