The largest survey of children in Europe and North America has revealed a worrying level of hard drinking and underage, unprotected sex among British youngsters.

The study of 11 to 15-year-olds in 35 countries found that 40% of 15-year-old girls in England and Wales had had sexual intercourse - twice as many as the average - and only 71% of those in England and 64% in Wales had used a condom. The figures put both countries second only to Greenland, where 79% of girls are sexually active.

The survey by the World Health Organisation also found that England and Wales ranked worst for alcohol abuse, with 58% of 15-year-old boys in Wales and 54% of girls drinking on a weekly basis, followed by 56% of boys and 49% of girls in England. Only 21% of American boys and 11% of girls drank. Not only were British youngsters drinking more often, they were also drinking more.

"This is something we need to address," said Candace Currie of the University of Edinburgh, who coordinated the research. "It is so embedded in our culture. You could say how can you tackle that, when the adult habit of alcohol use is as it is in this country.

"Even where some other countries have high rates of drinking they are not drinking to the point of being drunk. It is something about the way we drink that is a concern."

The study, which is conducted every four years, interviewed more than 160,000 children over 2001-02.

Behind the geographical idiosyncrasies - Scottish children drink the second highest proportion of fizzy drinks after Israel, and only 12% of boys in Malta clean their teeth every day - is a picture of an increasingly idle generation, taking risks with their health.

Less than half of the children in each country and region did the recommended amount of exercise of an hour or more five days a week. Girls were consistently less physical than boys. Around a quarter watched television for four or more hours a day, with one in seven spending more than three hours a day at the computer. By the age of 15, almost one in four were smoking and 29% were drinking on a weekly basis.

The UK fared better on smoking where rates have fallen, particularly in Scotland where 16% of 15-year-olds now say they smoke every week. Greenland has the worst rate with 67% of girls and 57% of boys smoking every week, while Canada has the best with only 15% of 15-year-old girls using cigarettes.

Dr Currie said the sexual activity rates in the UK were a concern but had stayed fairly static since the last report. The average across all regions was 20% for girls and 28% for boys. In England, 36% of boys said they had had sex, 4% less than the girls. In Wales, 29% of boys had had sex, compared to 33% in Scotland.

"They are sexually active but not really protecting themselves," Dr Currie said. "That is something we should be doing something about." Cannabis use was also found to be common, with 22% of 15-year-olds having tried it, and 8% using it regularly in the last year.

America had the fattest children, with 35% of boys either overweight or obese, compared to an average of 14% across all regions. In the UK, 23% of Welsh boys and 17% of Welsh girls were overweight or obese, compared with 17% for boys in England and 13% for girls.

On body image, girls considered themselves to be in worse shape than they were, with a third believing they were too fat, compared to a fifth of boys.

Vivian Barnekow Rasmussen of the WHO said the study had thrown up disturbing trends but would be an invaluable tool for health and other professionals to address the issues facing the new generation. "The youngsters know everything about the dangers _ but they don't relate it to themselves."