The figures come just a day after research was released by the same organisation claiming three in every four men and two in every three women will be overweight by 2030.
With 23.1% of youngsters in the under-five age group classed as overweight or obese, the UK was second only to Ireland, which registered 27.5%.
Britain was followed by Albania (22%), Georgia (20%), Bulgaria (19.8%) and Spain (18.4%).
Kazahkstan had the lowest obesity rate (0.6%), while other low-prevalence nations included Czech Republic (5.5%) Belgium (7%) and Sweden (8%).
The European Congress on Obesity heard the research and claimed that with children reliant on others for food, education on healthy eating is key, especially for pregnant women.
Dr João Breda, World Health Organisation’s regional office for Europe, who was involved in the first study, said: “Evidence suggests that early intervention, before five years of age, is necessary if the trajectory to overweight in children is to be arrested and action needs to be taken to have consistent surveillance on this specific population.”