Police discovered more than 20 cannabis farms and factories in the UK every day last year, seizing drugs worth up to £100 million, figures showed on Monday.

Criminals were attempting to reduce risk and minimise potential losses by employing a large number of so-called "gardeners" to manage smaller farms across residential neighbourhoods, the report by the Association of Chief Police Officers (ACPO) said.

A total of 7,865 farms were found across the UK in 2011/2012, an increase of 15 percent from the 6,866 found in 2009/2010 and up more than 150 percent from the 3,032 found four years ago, the ACPO study found.

The highest number of farms - 936 - were found in West Yorkshire, equivalent to 42 factories per 100,000 people.

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

Roughly 1.1 million plants with a street value of £207.4 million have been seized in the two years since the last ACPO report.

The number of offences related to cannabis production also increased from 14,982 in 2010/11 to 16,464 last year.

Scotland Yard Commander Allan Gibson, ACPO's lead on cannabis cultivation, said the practice posed a "significant risk" to Britain.

He added: "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."