More than a third of British teenagers are overweight or obese.
One in five 14-year-olds are obese, putting them in danger of diabetes and heart disease as adults.
Teenage girls are most likely to pile on the pounds during puberty, according to a study of more than 11,000 children born at the turn of the millennium.
The study found that children born after the 1980s are up to three times more likely to be overweight than those born before that decade
Among those of a normal weight at age 11, almost one in seven became overweight by the age of 14, despite the link between weight gain and low self-confidence and depression.
The statistics were described as ‘alarming’ by Royal College of GPs chairman Professor Helen Stokes-Lampard, who said it threatened the health of future generations.
The findings show children who were not breastfed, whose mother had less education or did not live in a house they owned, were more likely to be overweight or obese.
Co-author Professor Emla Fitzsimons, from University College London, said: ‘As members of the millennium generation reach early adolescence, rates of obesity and overweight remain a public health concern.
‘These findings show that although rates of excess weight have stabilised since age 11, there is still a worryingly high proportion of young people in this generation who are an unhealthy weight.’
A group of children born in 2000 and 2001, known as the Millennium Cohort, have been followed for years by researchers to monitor obesity among those nicknamed ‘generation Z’.
Royal College of GPs chairman Professor Helen Stokes-Lampard described the statistics as ‘alarming’
The first cause for alarm came between the ages of seven and 11, when the percentage of children who were overweight or obese leapt from 25 to 35 per cent.
The latest study shows the crisis is continuing, with a larger proportion of girls having become overweight in their teenage years.
UCL researchers found 15 per cent of girls who were a normal weight aged 11 are now overweight, compared to 10 per cent of boys.
Almost 40 per cent of children whose mothers had a GCSE or lower qualification were found to be overweight.
Just 14 per cent of children whose mothers had a degree or higher qualification fell into that group, but campaigners say junk food advertising and a lack of education are also to blame.
Prof Stokes-Lampard said: ‘This alarming finding that one in five young people are obese by the age of 14 should strike a chord in all of us.
Researchers spoke of an absolute crisis after childhood obesity has risen across the country
‘The stark truth is that overweight and obese children face a lifetime of health-related problems, including increased risk of conditions such as cancer and diabetes if their weight is not addressed.
‘As well as threatening the health of our future generations, obesity-related conditions cost the NHS billions every year, which affects everyone. The earlier we tackle this in our patients, the better.’
The UCL study found that children born after the 1980s are up to three times more likely to be overweight than those born before that decade.
Children who were large babies are more likely to be overweight teens, but being breastfed for at least 90 days and growing up in a house their parents owned appeared to protect them from obesity.
The authors also found differences between white and black teenagers, with 48 per cent of young black people overweight, compared to 34.5 per cent of white teenagers.
Dr Alison Tedstone, chief nutritionist at Public Health England, said: ‘We’re working with industry to make food healthier, we’ve produced guidance for councils on planning healthier towns and we’re delivering campaigns every day encouraging people to choose healthier food and lead healthier lives. It’s taken many years for us to reach this point and change will not happen overnight.’