A Chubby baby has been seen for years as the epitome of good health, but new research on the way children grow is set to overturn the belief that big is beautiful.

A six-year study by the World Health Organisation into how more than 8,000 children across different continents put on weight in their first years has revealed that those given the best start in life - by being breast-fed and having non-smoking mothers - ended up significantly lighter than the optimum weights suggested by current guidelines.

Child growth charts are now based largely on studies that mostly looked at babies fed on formula milk. The new work suggests that for years experts across the world have been significantly overestimating how many pounds babies should weigh.

This means many toddlers thought to be healthy could actually be overweight or even obese, and breast-feeding mothers who are told their babies are underweight may find that the infants are exactly the right size.

The research studied babies across America, Norway, Ghana, India, Oman and Brazil as they grew up, measuring their height, weight and their milestones in their progress such as crawling and walking.

All were breast-fed for six months by middle-class mothers who did not smoke. The study showed that despite the differences in nationality and genetic background, the babies all gained weight at a remarkably similar rate, piling on the upounds while they were breast-fed and then slowing their weight gain as they were weaned at six months.

But significantly, they ended up lighter at one, two and three years of age than if they had been formula-fed.

The rate of weight gain in childhood is a key determinant of whether teenagers and adults develop obesity, heart disease and diabetes later in life.

The results of the study, which will be presented in two weeks in London, will reinforce calls for a rewriting of the international growth charts on which advice to parents is based.

Mothers who are now told their children are slightly underweight may actually discover that they have a child who is a completely right weight for their age, once the new data is taken into account.

It could mean that the current references used by health visitors and doctors to decide whether or not children are the correct weight are out by between six and seven per cent.

This would mean, for example, that a one-year-old girl who weighs 10 kilos (22lbs) and is considered the perfect weight for her height, should probably be around 600 grammes lighter.

Dr Mercedes de Onis, the WHO study co-ordinator, said: 'The children who are breast-fed have more rapid growth at the beginning of life - in the first few months - but then a smaller rate of growth. They have different sleeping patterns, different metabolic rates and they are thinner.'

If countries such as Britain started to adopt the new figures for measuring children, it would throw more children into the overweight and obese categories,' she said.

'The generation of children who are raised now will be the heaviest that have ever lived. It is not something we can ignore.'

She was backed by Professor Ricardo Uauy, of the London School of Hygiene and Tropical Medicine, who advises the International Obesity Taskforce on child nutrition. 'The figures show that the differences are quite marked between these findings and what we have been taking as the norm, based on formula-fed children,' he said.

'If we have been measuring them using the wrong standards, then we have actually been promoting weight gain, and heavier children.

'With these figures, a child who currently would appear to be slightly overweight might now be obese. Potentially, we may have been seeing children as a normal shape when they are not. It's very worrying particularly because very often mothers are told to switch to formula because their children are seen as underweight.'

In the past, 'a bonny' baby was seen as one who put on a lot of pounds in the first year, partly because this was a defence against the infectious diseases that used to sweep through communities.

'Perhaps when whooping cough or diarrhoea was a real hazard, there was a case for having heavy babies. But we have different needs now, and there are consequences to being too big,' Uauy said.

Breast milk left the children with a natural immunity against infections and allergies, and they suffered fewer ear infections and stomach upsets. And in the long term, they were less likely to become obese or develop heart disease.

The National Childbirth Trust wants a major government campaign to make breast-feeding more acceptable because although 72 per cent of UK mothers start by feeding their babies like that, most give up quickly.

Rosie Dodds, its policy officer, said: 'We should be telling parents that breast-feeding is the norm and formula feeding has drawbacks. It is common that women get to six or eight weeks and have some difficulties. That's when they need support.'

She added: 'New WHO figures may make people reassess how we measure weight and what kind of message to give new parents.'