Officers in Newham want the public to alert them to anything suspicious because their resources are stretched after the terror attacks.

Each factory houses high-grade plants, known as skunk, capable of producing about �500,000 of the drug every year.

They are often run by criminal gangs and police think the area has been targeted due to cheap property prices.

The gangs usually tap into power supplies for other properties to run the special lights needed for the plants to grow.

Curtain clue

Detective Inspector Dave McKelvey told BBC News that 121 factories had been discovered in the borough.

He said: "Most of the premises that are being used are terraced houses, usually two or three bedroomed and there won't be much movement to and from the address,

"You should also be looking for the curtains to be drawn on a permanent basis and at night you are looking for very bright lights seeping through the curtains."

Mr McKelvey said there have been problems with the factories in other London boroughs and surrounding counties.