Children are packing in so much sugar at breakfast that half their daily allowance has already been eaten before school, Public Health England says.
It warns that sugary cereals, juices and spreads are all damaging to health.
Rotting teeth, ballooning waistlines and long-term health problems like type 2 diabetes are caused by unhealthy diets.
Officials are encouraging parents to use an app that reveals the sugar content of food and drink.
Around a quarter of five-year-olds have tooth decay and nearly a fifth of children are already obese by the time they leave primary school.
Sugar is the prime culprit with the National Diet and Nutrition Survey showing four- to 10-year-olds consuming twice as much sugar as they should be.
A survey of 200 parents with children aged four to 10 revealed the problem starts at breakfast.
It found children were eating more than 11g of sugar or nearly three sugar cubes, on average, at breakfast alone.
That adds up to more than 1,000 cubes of sugar at breakfast over the course of a year.
Source: BBC Health