It is what failed dieters have long suspected: fat people really can't keep the weight off.

Scientists have confirmed that the majority of overweight people who try to lose weight either by cutting calories or exercising will return to their former size.

Fewer than 10 percent of the 12million Britons who go on a diet each year succeed in losing significant amounts of weight and most of those who do put it all back on again within a year.

The study of 25,000 people provides further evidence of the prevalence of ‘yo-yo dieting’ where slimmers get into a cycle of losing weight and regaining it.

The scientists, from the Medical Research Council’s National Survey of Health and Development, have concluded it is better to avoid getting fat in the first place.

They followed 5,362 men and women from their birth in 1946 and 20,000 from birth in 1958, measuring their weight and blood pressure and assessing their lifestyles. The researchers found both groups began gaining weight in the 1980s and have steadily increased in size ever since.

Dr Rebecca Hardy, the council’s programme leader on body size, said: ‘Once people become overweight, they continue relentlessly upwards. They hardly ever go back down.

‘A few lose weight but very few get back to normal. The best policy is to prevent people becoming overweight.

‘For men [weight] goes up steadily through life. For women it starts slowly and accelerates in the mid-thirties.’

But the study’s findings do not mean dieting is pointless, as eating less and taking more exercise can increase fitness and lower blood pressure.

(Read by Renee Haines. Renee Haines is a journalist at the China Daily Website.)