Media playback is unsupported on your device Media caption Cdr Allan Gibson of the Metropolitan Police: "They've brought in lighting equipment, ventilation equipment"

An average of more than 21 cannabis factories were found daily in Britain last year, police chiefs say.

Officers confiscated marijuana in the UK with a street value of £100 million, according to an Association of Chief Police Officers (Acpo) report.

The number of farms discovered increased to 7,865, more than doubling in four years.

The UK is at "significant risk" from criminal gangs who cultivate cannabis on a commercial scale, says the report.

It estimates that the number of recorded cannabis production offences in the period from April 2011 to March 2012 will rise to 16,464, up from 14,982 in 2010-11.

In the last two years, police forces have seized 1.1 million cannabis plants.

Areas with most cannabis farms per 100,000 people South Yorkshire 64

West Yorkshire 42

Avon and Somerset 40

Merseyside 30

Lancashire 29

West Midlands 25

Northamptonshire 24

Staffordshire 20

Cheshire 19

Nottinghamshire 19 Source: projected Acpo figures for 2011/12

Based on a street value of £134 per ounce, the drugs are valued at £207 million.

Organised crime

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

The report also recorded an increase in robberies, burglaries and violence - including the use of firearms - linked to cannabis farms.

There is evidence of "taxing", or stealing of crops, while debt bondage is being used to control some cultivators.

Criminals are spreading risk, to reduce detection and financial losses, by paying a large number of "gardeners" to manage smaller crops in residential areas.

The study notes a shift from cannabis farms in commercial and industrial properties to "multiple site" small scale factories.

Increasing numbers of organised crime groups are diverting into this area of criminality... Allan Gibson , Scotland Yard police commander

It also says that with the economic downturn and a reduction in amounts supplied by drug dealers, the number of personal use cultivation offences is rising. Police intelligence suggests the purchase of seeds and hydroponic equipment (for growing the plants without soil) is on the increase.

More offences

More farms were found in the West Yorkshire force area - 936, or 42 factories per 100,000 people - than any other in the country.

But South Yorkshire had 64 farms for every 100,000 people, the highest per capita in the UK, with 851 farms.

Some 663 farms were found in the West Midlands or 25 per 100,000 people, while the Metropolitan Police had 608 farms, or eight per 100,000 people.

The highest rise in the number of farms since 2009/10 was recorded in Devon and Cornwall, where the number rose 1,664% per cent from 11 to a projected total of 183.

Fife saw a 488% increase from eight farms to a projected 39 for 2011/12.