France has by far the highest proportion of clinically underweight women in Europe, but only half of them think they are too thin, according to a new study.

In other European countries the opposite is true: the number of women in Britain, Spain and Portugal, for example, who see themselves as seriously skinny easily outstrips the number who actually are.

"This shows that what people consider an ideal weight in France is lower than in other countries," said the study's author Thibaut de Saint Pol, a researcher at France's National Institute of Demographic studies, which published the study on Wednesday.

"If a French person who feels fat were to go to the United States," - which has much higher rate of obesity - "he probably wouldn't feel fat anymore," he said.

The study also reveals a big gap, both objective and subjective, between sexes.

In western Europe, the mean weight of men in every country except France and The Netherlands tips the scales into the "overweight" category, according to World Health Organization (WHO) standards.

By contrast, in only three nations do women join the men in crossing that line: Britain, Greece and Portugal. And only among the Dutch does one find more overweight women than men.

France is the one country in which both sexes are solidly in the "normal" weight bracket, and the only one in which more than five percent of women are officially "underweight".

The universal standard introduced by the WHO for assessing weight is the Body-Mass Index (BMI): one's weight in kilograms divided by the square of one's height in meters.

A BMI of 25-to-30 indicates being overweight, while above 30 means one is obese. The range of normal weight is 18.5-to-24.9.

The proportion of overly thin women in France has long been the highest in Europe, but has shrunk from 8.5 percent in 1981, to 7.8 percent in 1992, to 6.7 percent in 2003, according to once-a-decade national surveys.

In that same period, the proportion of underweight French men held steady at just under two percent.