In this age group, 29.2% of the British population are excessively heavy. Just over 8% of the girls meet the clinical definition of obesity, having a Body Mass Index (BMI) of 30 or above.
Of the 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% lower prevalence.
The study confirms previous research that has shown roughly a 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.
Commenting on the findings, published in The Lancet medical journal, Professor John Newton, of Public Health England, said: "The levels of obesity in the UK - and indeed, across the world - are of great concern. That is why Public Health England is putting in so much effort to attempt to reverse the trend.
"The challenge of obesity is at the heart of current debate about the health of the nation and we are working closely with local authorities, the NHS and the voluntary and community sector to tackle this complex issue."
The study 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 just 10 countries - the US, home to more than 13%, China, India, Russia, Brazil, Mexico, Egypt, Germany, Pakistan and Indonesia.
In the US, about a third of the adult population is now obese, while in Australia 28% of men and 30% of women fall into this category.
Of Britain's neighbours in western Europe, tiny Andorra with a population of just over 78,000, had the best rates with 10.6% of adult men and 7.2% of women classified as obese.
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, widespread, and have arisen over a short time. However, there is some evidence of a plateau in adult obesity rates that provides some hope that the epidemic might have peaked in some developed countries and that populations in other countries might not reach the very high rates of more than 40% reported in some developing countries."
She added: "Our analysis suggests that the UN's target to stop the rise in obesity by 2025 is very ambitious and is unlikely to be achieved without concerted action and further research to assess the effect of population-wide interventions, and how to effectively translate that knowledge into national obesity control programmes.
"In particular, urgent global leadership is needed to help low-and middle-income countries intervene to reduce excessive calorie intake, physical inactivity, and active promotion of food consumption by industry."
Other key findings include:
:: In the developed world, men have higher rates of obesity than women, while in developing countries the opposite is true.
:: The greatest increase in rates of excess weight and obesity occurred between 1992 and 2002, mainly among people aged 20 to 40.
:: On the Pacific island of Tonga, more than half the population of men and women is obese.
:: In the western European region, which includes Israel, obesity levels among boys range from 13.9% in Israel and 12.9% in Malta to just over 4% in the Netherlands and Sweden.
Definitions of overweight and obese are based on BMI, which is obtained by dividing a person's weight in kilograms by their height in metres squared.
A BMI of 25 to 29 is considered "overweight" while 30 and above is classified as "obese".
Commenting on the findings, Professor Klim McPherson, from Oxford University, wrote in The Lancet: "An appropriate rebalancing of the primal needs of humans with food availability is essential, which would entail curtailing many aspects of production and marketing for food industries. To prevent unsustainable health consequences, BMI needs to return to what it was 30 years ago."
Research had shown that reducing BMI to 1980 levels in the UK would require an 8% reduction in consumption across the country, costing the food industry roughly £8.7 billion per year.
Prof McPherson added: "The solution has to be mainly political and the questions remain, as with climate change, where is the international will to act decisively in a way that might restrict economic growth in a competitive world, for the public's health? Nowhere yet, but voluntary salt reduction might be setting a more achievable trend. Politicians can no longer hide behind ignorance or confusion."
Sir Stephen O'Rahilly, Professor of Clinical Biochemistry and Medicine and Director of the Metabolic Research Laboratories at Cambridge University, said: "It seems unarguable that excessive caloric consumption is playing a major role in driving the increasing prevalence of obesity.
"It is helpful to get a well-informed semi-quantitative estimate of the sorts of overall reduction in calorie ingestion we would need to return obesity levels to those found in the 1980s. However, linking this caloric decline to an inevitable decrease in the profitability of the food industry seems less compelling, and might even backfire.
"It would be optimal if the food industry became helpful partners, not adversaries, in countering the 'obesity epidemic'. It is less likely to do so if hectored about substantially slashing its profits."
A Department of Health spokesperson said: "As this study shows, obesity is a problem faced by many countries across the world - and while we know there is much more to do, in England we are making progress, with child obesity now at the lowest level since 1998. We have given local authorities a dedicated budget of more than £5 billion to tackle public health issues like this.
"The World Health Organisation has said our salt reduction work is world leading. We are working with industry and have seen successes in the voluntary reduction of fat, sugar and salt in foods, which helps people to make healthier choices about the food they buy."