"The behaviours modelled by the chefs could lead to incidences of food-borne illness, especially among those who mimic their behaviours at home," said the team from Kansas and Tennessee State Universities in the Journal of Public Health.
"These chefs ... either ignore food safety or at best demonstrate only very limited positive behaviours."
Although all the cooks washed their hands at some point during the shows, 88 per cent failed - or were not shown - to do so after handling raw meat. Around one in five was guilty of touching their hair while preparing food, while 21 per cent licked their fingers. Miss Lawson's hand-washing was deemed inadequate - as well as her habit of dipping her finger into dishes while cooking.
Of all the cooks, Miss Berry was the most hygienic. However even she was seen wiping her nose and fiddling with her grandson's hair.
Last night Dr Lisa Ackerley, of the Royal Society for Public Health, said: "It is irresponsible for famous personalities ... to ignore safe practice because people will inevitably copy what they see."
A spokesman for Mr Oliver said: "Jamie's programmes are all about getting people excited and engaged with cooking and clearly food hygiene is an important part of this."
Miss Lawson declined to comment, while Miss Berry was "unavailable due to filming commitments".