A cannabis factory capable of returning a £1m-a-year profit has been closed down by police in Norfolk. More than 3,000 plants - the biggest haul in the county in recent years - were found when they raided a converted pig shed.

A cannabis factory capable of returning a £1m-a-year profit has been closed down by police in Norfolk.

More than 3,000 plants - the biggest haul in the county in recent years - were found when they raided a converted pig shed outside Attleborough on Saturday.

Officers from the force's western area had expected to find a small-scale drug- growing outfit but were shocked by the high-level operation they uncovered.

Forensics experts have been examining the premises and removing the plants. It is thought the factory was set up in mid-autumn and had already produced one harvest of skunk, one of the strongest strains of cannabis available.

While traditional cannabis farms can produce just one crop a year, the hi-tech hydroponics systems installed meant this farm could produce three or four.

The building was split into eight cultivation zones and in one a crop was almost ripe for harvest and could have been ready for sale on East Anglia's streets within weeks.

Det Con Steve Hamilton, an expert in drugs farms, said: “This is clearly the work of an organised group of individuals who knew what they were doing and had the resources to invest into it.

“Most of the equipment used can be bought legally over the internet but it is not cheap. The light bulbs alone would have cost about £40 each. They would have also had their own electrician to wire the whole place up.

“The size of the factory suggests somebody would have been visiting the property several times a day to monitor and tend to the crop.”

The growing rooms were hidden behind fabricated walls so visitors would not become suspicious. The criminals behind the operation had put up an initial outlay of at least £15,000 to set up the factory.

As well as the hydroponics lights used to increase crop yield, the factory was fitted with its own electricity generator and extractor fans used to divert the distinctive smell away from the premises.

Det Con Hamilton added that forensic calculations to determine the precise size and value of the crop were still being carried out. But he said estimates suggested the operation would produce at least £1m of the Class C drug each year.

So far, nobody has been arrested, but Det Sgt Mark English, who is leading the investigation, said a large amount of intelligence had been gathered. The investigation will rest upon forensic evidence and work to establish who the building, owned by a local farmer, was leased to.

Det Sgt English said: “We initially sent a small team of four officers to search the premises. When we were granted a warrant we had no idea we would uncover one of the largest grow-ops in Norfolk's history.

“We have taken the decision to gather as much evidence as possible before making any arrests. One possibility, as we have seen in other cannabis farms, is that the people responsible for the day-to-day running of the factory were not responsible for funding and organising the whole operation.”

Det Chief Insp Neil Firm urged members of the public to come forward with any information about such operations.

He said:“Any incidents of suspected cannabis production should be reported to the police - your information will be acted on.”

Anyone with information about any cannabis production should call Norfolk police on 0845 456 4567.