The West Midlands police area showed one of the highest ratios of forces for cannabis factories per 100,000 population in 2009/10.
The force showed 30 factories per 100,000 population, said the Association of Chief Police Officers (Acpo).
More than 1.3 million plants worth an estimated £150 million were recovered across the UK in the last two financial years, Acpo said.
In 2009/10, police seized almost 750,000 plants with an estimated yield of £85 million, compared with more than 500,000 plants with an estimated yield of £65 million the year before.
And the UK is producing so much cannabis that "there is now a market for exportation", the police chiefs warned. But their report noted there was "no intelligence or evidence to suggest that this is happening".
Across the UK, police discovered 11 cannabis factories per 100,000 people in the 2009/10 financial year, compared with five in 2007/08 and eight in 2008/09.
The highest number of factories (896) were found in the West Yorkshire force area, equivalent to 41 factories per 100,000 population, up 141% from 17 factories per 100,000 in 2007/08, the Acpo figures showed.
Greater Manchester Police found 32 factories per 100,000 in 2009/10, the second highest ratio of any force, followed by the West Midlands (30), and Avon and Somerset (25).
The North Yorkshire force recorded the highest rise in the number of factories found, up 1,200% to 13 last year from just one in 2007/08 - but the force does not experience a large number of factories compared with other areas, the report found.