More than 20 cannabis farms and factories were discovered by police every day last year as they seized drugs which could sell for £100 million on the streets, figures showed today.
Senior police chiefs said the size and scale of the farms were reducing as criminals producing cannabis were spreading the risk and minimising losses by employing a large number of so-called gardeners to manage small sites across multiple residential areas.
Over the two years since the last report by the Association of Chief Police Officers (Acpo), some 1.1 million plants have been seized with a street value of £207.4 million.
A total of 7,865 farms were found across the UK in 2011/12, up 15% from 6,866 in 2009/10 and more than a 150% increase from the 3,032 identified four years ago, the study by the Association of Chief Police Officers (Acpo) found.
There has been a "move back to the use of residential property" and dismantling factories was seen as "a short term solution, with missed opportunities for further investigation into potentially linked factories", the police chiefs said.
The number of offences related to cannabis production is also increasing, up from 14,982 in 2010/11 to 16,464 last year.
Scotland Yard Commander Allan Gibson, the lead on cannabis cultivation for the police chiefs, said:
The highest number of farms (936) were found in the West Yorkshire force area, equivalent to 42 factories per 100,000 people, the Acpo figures showed.
But South Yorkshire had 64 farms per 100,000 people, the highest ratio in the UK, with 851 farms.
The two forces were followed by other heavily-populated force areas, including West Midlands (663 farms, or 25 per 100,000 people), the Metropolitan Police (608 farms, or eight per 100,000 people) and Avon and Somerset (653 farms, or 40 per 100,000 people).
But the Devon and Cornwall force recorded the highest rise in the number of farms since the last report in 2009/10, with the number of farms identified rising 1,664% from 11 to 183 (11 farms per 100,000 people).