An inferno which devastated a polystyrene factory accidentally started in one of the machines on the shop floor, fire investigators revealed today.

An inquiry team which was at the scene an hour into the incident has interviewed staff at Styrene Packaging and Insulation Limited, in the Park House Industrial Estate in Low Moor.

The building went up in flames on Wednesday evening and parts of the factory were still smouldering today.

Speaking from inside its ruins, Brian McKeating, Station Commander of South Elmsall station, near Pontefract, who is leading the investigation, said: "We have spoken to five witnesses who were on the shop floor when the fire started and we now know it originated in a machine that cuts the polystyrene.

"We have yet to determine what went wrong with that machine, but a specialist team will be looking at it.

"There is no suspicion of anything doubtful or suspicious - it is clearly accidental.

"At the moment we are in the shell of the building, which has no roof, and there is a lot of tangled debris and metalwork that came down with the ceiling."

Most of the building collapsed, and remaining walls are unsupported, which made it impossible for fire crews to get inside and put out the remaining pockets of fire.

Station Officer Martin Speed, at Odsal fire station, said: "We are checking the site every few hours to make sure the smouldering fires are not growing in size, but we cannot close the job until it is all out."

Directors of Styrene would not comment on the cause of the fire.

A spokesman for the British Plastics Federation said he believed that the fire would not have been caused by the polystyrene itself.

"Expandable Polystyrene (EPS) is one of a number of plastics materials which are approved for use in the building industry and, as such, EPS products fulfil the fire safety requirements of the building regulations," he said.

"Fire retarded forms of EPS are classified in many countries in the best class for combustible materials. There are no known instances where the use of EPS has itself been the cause of a fire."

The fire site backs on to a housing estate, but public health chiefs believed there was no cause for concern.

Brian Anderson, Bradford Council environmental health manager, said: "There are some very fine sooty deposits in the area closest to the fire which are typical of any fire of this size.

"Given the nature of the fire much of the pollution is gaseous, so I would expect to find very little evidence of other fallout from the fire."

Dr Ruth Gelletlie, of the Health Protection Agency in Bradford, which incorporates public health, was satisfied it was unlikely there was a risk to the public, but warned people to peel or scrub all home-grown fruit and vegetables before eating.