Police say they can use sales-trend information by DIY stores to track down people buying suspicious amounts of fertilisers, fans, lighting and venting, and other equipment used to grow cannabis.

The project means police can find out more about where drug manufacturers are operating and under the gaze of the store CCTV systems.

The unusual step comes as officers' concerns grow about the link between drugs farms and serious organised crime, such as people trafficking.

Investigators are also worried that the potency of cannabis in Britain is stronger than a decade ago and could lead to more health problems and escalating drug use.

Insp Steve Stebbings, of Lancashire's Serious and Organised Crime Unit, said: "We are hoping to identify individuals who purchase the type of products we are looking at.

"We have produced a leaflet to give out to DIY stores in the county. We appreciate innocent customers buy these products every day and they have nothing to fear.

"We will be looking at factors such as unusual amounts of fertiliser sold in the middle of winter, or high cash payments for big quantities. We won't follow up on every sale, just those that trigger suspicion. Potent varieties of cannabis, known as skunk, now makes up around 80% of British cannabis according to the Home Office.

"Its potency has increased around three times in the last decade.

"Compared to 10 years ago it contains more of a chemical called THC but less of another compound that helps to counteract psychotic episodes triggered by THC."

Lancashire has seen a spike in cannabis farms over the last two years. Many have been in converted suburban homes, like a set-up of 400 plants found in Lytham Road, Fulwood, Preston, in December, and an £800,000 find in a £1m rented mansion on posh D'Urton Lane in Broughton, Preston, in 2007.

And 1,000 cannabis plants – worth about £600,000 – were found in Oakenclough Mill, near Garstang, in September.

It is suspected that foreign people-trafficking gangs are behind some of the farms, and police are also working with utilities firms to identify suspicious levels of domestic energy use.

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