More than 20 cannabis farms and factories were discovered in the UK every day last year as officers seized drugs that could sell for £100 million (€133 million) on the streets, figures showed.

Commercial cannabis cultivation continues to pose a significant risk to the UK

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.

In 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 (€255 million).

A total of 7,865 farms were found across the UK in 2011/12, up 15 per cent from 6,866 in 2009/10 and more than a 150 per cent increase from the 3,032 identified four years ago, the study by Acpo found.

There has been a “move back to the use of residential property” and dismantling factories was seen as “a short term solution”.

There have been “missed opportunities for further investigation into potentially linked factories”, the police chiefs said.

The number of offences related to cannabis production has also increased, 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: “Commercial cannabis cultivation continues to pose a significant risk to the UK.”