Sugar-laden breakfasts mean that the nation's children are consuming half of their daily sugar allowance before they even start school, health officials have warned.
The average English child eats the equivalent of three cubes of sugar (11g) every morning for breakfast, Public Health England (PHE) said.
Sugary cereals, fruit juice and some spreads are to blame, according to the health body's new Change4Life campaign.
By the end of the day, children are consuming three times more than their recommended daily allowance of sugar, according to information gathered from the National Diet and Nutrition Survey.
The recommended daily maximum is five cubes of sugar for four to six-year-olds and six cubes for seven to 10-year-olds.
Data from the National Child Measurement Programme shows that more than one in five children start primary school overweight or obese, rising to more than a third by the time they start secondary school.
PHE said that it has launched a free new app - Be Food Smart - which helps highlight how much sugar, saturated fat and salt can be found in everyday food and drink that children consume.
The app works by scanning the barcode of a product so parents can make healthy choices as they shop.
Dr Alison Tedstone, chief nutritionist at Public Health England, said: "Children have far too much sugar, and a lot of it is before their first lesson of the day.
"It's crucial for children to have a healthy breakfast, but we know the mornings in a busy household can be fraught.
"That's why we've developed our Be Food Smart App, taking some of the pressure off parents and helping them to choose healthier food and drink options for their children."
Sara Stanner, science director at the British Nutrition Foundation, added: "We know a healthy breakfast can make an important contribution to children's vitamin and mineral intakes and its consumption has been linked to many positive health outcomes.
"There are plenty of healthier options available so we need campaigns like Change4Life to help busy parents make the right choices for their families."
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