Eighty-three percent of chicken sold in U.S. grocery stores may contain bacteria that cause foodborne illnesses, a consumer group said on Monday. That number is 34 percentage points higher than the rate it found three years ago.

Consumer Reports said tests on 525 chickens – including samples from leading brands Perdue, Pilgrim’s Pride Inc. and Tyson Foods Inc. – showed most of the poultry had campylobacter or salmonella, two of the leading causes of food-borne diseases.

"We think it’s really startling," said Jean Halloran, a policy director for Consumers Union, which publishes Consumer Reports. "It’s a very significant deterioration in food safety."

A spokesman with the U.S. Agriculture Department’s Food Safety and Inspection Service said the study was riddled with flaws such as a small sample size and uncertainty over the report’s methodology.

The Center for Disease Control and Prevention estimated the two bacteria, which can be spread through other avenues in addition to chicken, cause millions of illnesses and 700 fatalities annually.