A study of 1,000 girls found that breast development now begins on average a year earlier than 20 years ago - around the age of nine years and ten months.
The research underlines a long-term trend that has seen the average age at which girls start puberty falling sharply. In the 19th century it was around 15 - six years later than now.
Scientists yesterday spoke of the serious implications for girls' physical and emotional health. There are fears that early puberty could put girls at higher risk of breast cancer and heart disease because of the increased exposure to estrogen.
Dr Anders Juul, of Copenhagen University Hospital in Denmark, which carried out the research, said: "If girls mature early, they run into teenage problems at an early age and they're more prone to diseases later on.
"We should be worried about this, regardless of what we think the underlying reasons might be.
"It's a clear sign that something is affecting our children; whether it's junk food, environmental chemicals or lack of physical activity."
Richard Stanhope, an expert in hormonal disorders, said: "All the things we experience as teenagers are difficult enough to cope with, but when it happens at ten or 11 it is much worse."
Scientists believe the phenomenon could also be down to rising obesity, and changes in diet.
Another possible culprit is bisphenol A, a chemical found in the lining of tin cans and babies' feeding bottles. Dr Juul is testing blood and urine samples of the girls in the study to see if a link can be proved.
Boys have also been affected by the phenomenon. Choir schools have reported an increasing number of boys dropping out at 12 or 13 - because their voices had broken.
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