Parents have been accused of failing to help their children look after their teeth properly.
A survey found more than one in five under-fives were being left to brush their teeth unsupervised.
A quarter of parents wrongly thought children did not need to brush twice a day, and 67% thought brushing for one minute was enough - two is recommended.
The survey, of 1,000 people, was carried out by the British Dental Health Foundation (BDHF).
The results also showed that 23% of those surveyed thought there was no need for children to avoid fizzy drinks, even though they have been linked to dental damage.
Dr Nigel Carter, BDHF chief executive, said: "These results really are very worrying and help explain why around half of children under the age of five currently have tooth decay here in the UK.
"Teaching children good dental habits is vital.
"Not only has research shown that people who learn good habits as children are far more likely to carry them into adulthood, but taking bad habits into adulthood will cause gum disease and this has been linked to all manner of serious conditions including diabetes, strokes, heart disease and low birth weight babies."
Janet Clarke, of the British Dental Association, said teaching children how to brush their teeth was a vital part of oral hygiene.
She said: "Twice-daily brushing with fluoride paste should begin as soon as the baby teeth begin to erupt and will need to be performed or supervised by an adult until children are around six years of age.
"Parents have a crucial role to play in encouraging their children to get into good oral health habits from a young age."
The survey also found that 29% of UK adults suffered with bleeding gums and almost half of those simply ignored it or brushed their teeth more softly to avoid aggravating it.
Dr Carter said: "Bleeding gums are a sign of a poor oral healthcare routine and, if left, can lead to serious health problems.
"People need to brush twice a day with fluoride toothpaste, cut down how often they have sugary foods and drinks and visit the dentist regularly, as often as they recommend if they want to keep their gums and body healthy."