The Cadbury's chocolate factory at the centre of a food poisoning alert was infected with the same strand of salmonella four years ago, public health officials disclosed yesterday.

The Food Standards Agency disclosed the contamination of the Marlbrook plant in Herefordshire in 2002 as it released a damning verdict on Cadbury's justification for selling products containing reputedly low levels of salmonella. Herefordshire County Council, which covers Marlbrook, said its environmental health officers had not been informed about the contamination in 2002.

The independent Advisory Committee on the Microbiological Safety of Food also said that there was no safe level of salmonella and described Cadbury's scientific approach as old-fashioned and "unreliable". The company, part ofCadbury Schweppes, said that it had a danger level of 10 cells of salmonella per 100g chocolate and the tests revealed the presence of only 0.3 cells per 100g.

The developments will further tarnish the reputation of Cadbury, which JP Morgan investment bank believes could lose £25m as a result of the outbreak.

Twelve days ago, Cadbury announced the recall of one million chocolate bars and public health officers are testing a further 30 of the company's brands.

Health officials have confirmed that 31 people - with an average age of two - have so far been infected with the strain of Salmonella Montevideo, smvdX07, found in the bars, exposing them to stomach cramps and diarrhoea. The Health Protection Agency said that three had been treated in hospital.