Firefighters stumbled across a sophisticated underground cannabis factory thought to have produced harvests worth hundreds of thousands of pounds over a number of years.

The officers were dealing with a caravan fire at a farm when they uncovered the labyrinth of eight shipping containers where more than 300 mature plants were growing.

Sgt John Wallace, of Haywards Heath police station, said: "It's the largest find that I have worked on. Potentially, this will make some sort of impact on the amount of drugs available on the street."

Detectives were last night trying to trace a highly organised gang after the discovery at the 53-acre Leigh Water Mill Farm in Ansty, near Burgess Hill.

The burning caravan was less than 10m away from a shipping container perched on top of a further eight buried underground.

Firefighters used water to cool gas cannisters that had become heated by the fire.

A generator and power cables feeding into the containers were nearby. Fearing the generator would be at risk of the water, they investigated the container and discovered the factory.

Neighbours said there had not been any excavations in recent years but there was a lot of activity on the site 20 years ago.

They said people often came and went and night but thought that was because one of the occupants was a lorry driver.

One neighbour told The Argus: "I can't believe it. I knew something must have happened because the police put a big blue tent. We guessed it wasn't just arson."

Forensics teams are expected to remain at the scene well into the weekend. Safety concerns forced the police to cut the power supply and they are working in dark and damp conditions.

Sgt Wallace said the site had been used for several years. He told The Argus: "It's clearly historically been functioning to produce a significant amount of drugs. It appears to have been used some period, possibly a number of years.

"It's a relatively large scale factory for the production of drugs. The fact it's buried underground shows someone has gone to great lengths to conceal it. That to me is quite unusual and quite ingenious."

The land was bought in 1986 by widow Doreen Ellett, 83, who was unavailable for comment. Police believe she may have rented the land out.

Mrs Ellett's white Mercedes was wrecked in the fierce night-time blaze at her home in February.

No arrests have been made following the cannabis factory discovery and there is no suggestion that Mrs Ellett has broken any law. But police are looking to speak to anyone who has seen any unusual activity nearby.

Sgt Wallace said: "The site appears to have been deliberately generated for this purpose and the undertaking of such an operation would have been significant in terms of the moving of cargo containers and submerging them under earth along with arranging a strong generator power supply for heat and energy.

"This is clearly the undertaking of more than one person. I am keen to speak to any neighbours or regular road users who may have seen unusual activities in the area or significant excavation work. I would also like to hear from professional contractors who may have been involved with supplying materials to the area without knowledge of the real purpose."

Anyone with information should call Mid Sussex CID 0845 6070 999 or Crimestoppers anonymously, quoting Operation Gortin.