In this age group, 29.2% of the British population are excessively heavy. Just in excess of 8% of girls meet the clinical definition of obesity, having a Body Mass Index (BMI) of 30 or above.
Of 22 western European countries studied, only Greece is on a par with the UK as far as girls aged 20 and younger are concerned, with a 0.1 per cent lower prevalence.
The study confirms previous research that has shown roughly one-quarter of Britain's adult population to be obese.
Among adults aged 20 and above, 66.6% of men and 57.2% of women were either obese or overweight, while obesity affected 24.5% and 25.4% respectively.
The study, published in The Lancet medical journal, documented a "startling" surge in obesity and overweight rates in 188 countries around the world.
Between 1980 and 2013 the number of overweight and obese people rose from 857 million to 2.1 billion.
This represents an increase of 28% for adults and 47% for children.
But the research also found enormous variation around the world, with more than half the planet's 671 million obese individuals living in 10 countries - the US, home to more than 13%; China; India; Russia; Brazil; Mexico; Egypt; Germany; Pakistan and Indonesia.
Study leader Professor Emmanuela Gakidou, from the University of Washington, US, said: "Unlike other major global health risks, such as tobacco and childhood nutrition, obesity is not decreasing worldwide. Our findings show that increases in the prevalence of obesity have been substantial."