Marijuana with a street value of £100 million was seized as a result, according to an Association of Chief Police Officers (Acpo) report.

The number of farms discovered increased to 7865, more than doubling in four years.

The report said the UK was at “significant risk” from criminal gangs who were cultivating cannabis on a commercial scale.

Those criminals are now spreading the risk, to reduce detection and financial losses, by paying a large number of ‘gardeners’ to manage smaller crops in residential areas.

The study noted a move from cannabis farms in commercial and industrial properties to “multiple site” small-scale factories.

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