About half the cannabis consumed in Britain is grown in London

The city's escalating house prices are hitting the profit margins of gangs who operate such premises, the report from the Metropolitan Police Authority says.

Between 700 or 800 properties in London are equipped with hydroponic systems and lighting to cultivate large amounts of the Class C drug, it claims.

They are often run by Vietnamese gangs who rent or buy houses, police suspect.

The practice has become increasingly common across the country in recent years, and more than half the cannabis consumed in Britain is now believed to be grown in London.

Police tactics

Hi-tech police tactics such as using thermal imaging cameras to detect cultivation equipment have helped to cut the growth of factories, the report says.

However, the report also indicates that booming house prices also share some of the credit.

"Intelligence indicates that the number of new factories being established in London is gradually reducing (although there is a suspicion that house prices play a part in this)," the report says.

"The Metropolitan Police has employed a number of tactics including the use of thermal imaging, training on how to identify potential factories, and engaging with landlords and electricity companies to build intelligence pictures."