Cadbury failed to adequately assess the risk of salmonella in its chocolate, a report said yesterday.
The food giant apparently used "unreliable" methods which may have underestimated the level and likelihood of contamination.
Cadbury first discovered salmonella in some products, which were destroyed, in April 2002, the Food Standards Agency said.
The company recently pulled more than a million chocolate bars due to a salmonella scare.
The FSA's independent advisory committee on the Microbiological Safety of Food (ACMSF) said Cadbury's method of salmonella risk assessment could not be relied on for foods such as chocolate.
After meeting to discuss the issue on Friday, the committee yesterday said in a statement: "Based on the information provided, Cadbury appears to have used methods for product testing which the committee considered would underestimate the level and likelihood of salmonella contamination."
The company's risk assessment wrongly drew parallels between the threshold for salmonella infection and the threshold for infection by other micro-organisms which can be found in chocolate, the committee said.
In fact there is no minimum infectious dose for salmonella.
Other Cadbury's products made with contaminated chocolate crumb could be a cause for concern, the committee warned.
Committee spokesman Professor Tom Humphrey said: "We think the testing methods were insufficiently up to date and insufficiently sensitive.
"We think they made a mistake in assuming there was a safe level of salmonella in a product like chocolate. Our view is that there isn't."
Cadbury recalled more than a million chocolate bars across seven product lines on June 23.
The contamination - detected in January - was caused by a leaking pipe at the company's huge Marlbrook plant in Herefordshire.
Birmingham City Council's public health officers are carrying out tests on about 30 more of the company's products. Several other local authorities are involved in investigations.
Responding to the criticism, a spokesman for Cadbury said the company would improve its procedures.
He said: "We'll continue our dialogue with the regulators and will be improving our procedures in the light of their advice."