Babies who put on too much weight during the first four months of their lives could be condemned to a lifetime of obesity, according to the largest-ever study into infant weight gain.

While the bouncing baby has long been a cause for celebration, the findings suggest it should now be the subject of concern and regarded as a key to the epidemic of childhood obesity that is spreading the nation.

'These findings could lead to a whole new hypothesis regarding the cause of childhood obesity,' said Dr Nicolas Stettler, co-author of the report and associate professor at the Children's Hospital of Philadelphia and University of Pennsylvania.

'We have shown that early infancy constitutes a critical period for the establishment of obesity and, if these findings are true, we have established that the first months of life should be a special focus of intervention.'

The study, which tracked 30,000 children from birth, found that for each increase in weight of 100 grams per month above the expected average 700 gram increase, the risk of becoming an overweight seven-year-old increased by 30 per cent.

Published in the influential American journal Pediatrics , the study found a direct link between seven-year-olds suffering extreme obesity and babies who experienced rapid weight gain during the first four months of their lives.

Traditional critical points for monitoring weight are at gestation, three to six years of age and puberty. British experts are so concerned by the new findings that a similar project is to be launched in the UK.

'I am sure GPs are not aware of this at all: the emphasis is on looking for children who are not growing enough,' said Dr Mary Rudolph, from the Leeds Community NHS Trust whose recent study into childhood obesity found that one in three 11-year-old girls was overweight and one in 10 was clinically obese.

'Traditionally, paediatricians have not considered fat babies as being under any particular risk of turning into fat children, and hence fat adults,' she said.

Three out of five UK adults are now overweight or obese to the level at which their health is seriously threatened, while almost one in 10 children under four is obese.

According to Rudolph's own findings, the number of youngsters considered fat or obese has almost doubled in the last decade while a quarter of five-year-old girls are showing early warning signs of weight-related diabetes.

'This is fascinating research which shows we need to be as aware of overfeeding as we are of underfeeding babies,' said Pauline Emmett, senior research nutritionist for the Avon Longitudinal Study of Pregnancy and Childhood, also known as the Children Of The Nineties study, which is monitoring the health of nearly 11,000 babies as they grow into adults.

'We will start working on replicating the research into our own study immediately but I have no doubt that we will see their findings mirrored by our own, which is extremely worrying,' she added.

Studies have confirmed a range of factors that may predispose children to obesity, including maternal overweight, maternal gestational diabetes, high birth weight, formula rather than breast feeding, the early introduction of solid food and television viewing.

Experts are uncertain as to how a baby becomes overweight almost before it is weaned but the study's authors suggest genetic factors may be to blame.

Although obesity at seven years of age is too young to guarantee weight problems later in life, Stettler admits the two are likely to be linked.

'Prevention of obesity beginning in childhood is critical as excessive weight in childhood often persists into adulthood,' he said. 'Added to that, it is also a cause of non-weight related complications later in life, such as diabetes.'

But how and when to intervene to prevent obe sity in childhood is a difficult question and experts warn that placing a baby on a diet or restricted diet could cause irreparable damage.

Penny Gibson, a doctor for the Surrey and Hampshire Borders Trust and a consultant paediatrician and adviser on childhood obesity, said: 'These findings are very important but only as a way of helping us target who is more likely to become obese and why, but at the moment no one is recommending putting babies on a diet.'