The dead animals were found after farmworkers noticed that the entrace to the sett at Fairstead Hall Farm, near Whitham, was blocked.

The BBC's Robert Pigott: "In the wrong hands it could be deadly to people too"

Police say the gassing was done deliberately, probably with sodium cyanide, although they are baffled as to a motive.

The first police wildlife liaison officer to arrive at the scene was treated for the effects of breathing in the fumes.

RSPCA officers are searching for more dead animals on the farm, which is part of a large agricultural business owned by Lord Rayleigh.

'Agonising death'

An RSPCA spokeswoman said a number of badgers had been killed in Essex recently - although this was the most serious incident.

"This doesn't sound like badger baiting," said the spokeswoman. "When badgers are baited they are usually dug out and then chased by dogs.

"This sounds like someone deliberately trying to wipe out badgers because they are being a nuisance."

Local badger expert Don Hunford said: "They have been known to carry bovine tuberculosis but a survey in Essex has shown that badgers in this part of the world are not carriers.

Some farmers fear TB, carried by badgers, is the next BSE

"Culling is allowed in some very special circumstances but even then gassing is no longer the preferred method.

"The Ministry (of Agriculture) now prefer to trap and shoot badgers because gassing is not always effective and it can lead to the animals suffering a lingering and agonising death."

Police said staff on the estate were "horrified" by the incident and were co-operating with the investigation.

The public have been warned not to touch any tins or tubes which may have contained the poison, which could have been discarded in the area.

Sodium cyanide is normally used to control rabbits. It needs expert handling by users wearing protective clothing and masks.

The killing of badgers is illegal under the Protection of Badgers Act of 1992.

Culling is only being carried out in specially-selected "hotspots" by the Ministry of Agriculture, Fisheries and Food, to establish whether there is a link between badgers and tuberculosis (TB) in cattle.