Officers from the National Crime Squad (NCS) believe the factory, on a converted farm in Hurst Green, was capable of producing £1m worth of amphetamines every week.

After months of surveillance it was raided on Thursday afternoon, while production of the drug continued.

A woman and two men arrested at the farm and two men arrested in Streatham, south London, are being questioned at police stations in Hastings and Eastbourne.

Dangerous state

An NCS spokesman said two guns were also found at the address in Greyswood Road, Streatham, where one of the men was arrested.

Detectives said the factory could have produced up to 20kg of amphetamines - also known as speed - every week.

This is a major find and we believe we have disrupted a major drugs conspiracy

DCI Jim Warnock

Experts from the Forensic Science Service, who are helping to examine the factory said it was one of the most sophisticated they had ever seen.

Detective Chief Inspector Jim Warnock of the National Crime Squad said: "This is a major find and we believe we have disrupted a major drugs conspiracy.

"The plant was in a dangerous state, and although it is currently safe, we will be liasing with local authority environmental health officers to ensure a complete clean up as soon as possible."

A crime squad spokesman said the factory would have been a hazardous place for those entering it, because of fumes and the possibility of explosion.

The operation involved detectives from the National Crime Squad, the National Criminal Intelligence Service, the Forensic Science Service, the Metropolitan Police and Sussex Police.