However, Miss Packwood said the move was "controversial", adding: "We get a lot of grief about our fries."
The company has had more success with healthier boxed meals, which include a single piece of fried chicken surrounded by salad and rice for less than 500 calories. She said the best tactic was a "health by stealth" approach of gradually removing fat, calories and salt so that customers do not notice. Tesco group quality director Sarah Bradbury, also speaking at the conference, said the chain saw plummeting sales of its own-brand tomato soup "overnight" when it reduced its salt content 15 years ago in response to government targets.
She told how the brand put the salt back in and has slowly removed it since. The soup now contains 40 per cent less salt than 15 years ago.
Dr Alison Tedstone, chief nutritionist at PHE, insisted that "reformulating" products to make them more healthy will eventually be successful.
The organisation plans to set voluntary calorie caps on all meals served in restaurants and supermarkets in an attempt to combat obesity.
Dr Tedstone said she was not surprised that KFC's attempts to introduce healthier products had failed, because its customers were unlikely to be seeking healthy food.
However, she praised the chain's decision to stick with its thicker fries. "A small change in chips could have a massive effect," she said.