Babies who are likely to grow up obese can be identified when they are just six months old, scientists have established.

Infants who are overweight even when tiny babies are likely to be obese as primary school pupils, researchers said.

Doctors used simple Body Mass Index (BMI) measurements to single out infants destined to struggle with weight in later life.

Study leader Dr Allison Smego, from Cincinnati Children's Hospital, said: 'These children have a high lifetime risk for persistent obesity and metabolic disease and should be monitored closely at a very young age.'

Measuring babies at six, 12 and 18 months can predict children at risk of childhood obesity (file photo)

BMI compares someone's weight with their height, giving them a rough score to allow doctors to calculate whether someone is overweight for their height.

BMI is not usually calculated for babies under the age of two, because they grow at different rates.

But the Cincinnati scientists said that even with the different growth rates, the infants with the top 15 per cent of BMI measurements at six months, 12 months and 18 months were the most likely to be severely obese when they were six years old.

They said that identifying children who are at risk of obesity, even when they are just babies, could allow doctors and parents to stop them from growing up fat.

About 17 per cent of boys and 15 per cent of girls aged two to 15 in England are obese.

Experts have warned if the tide doesn't turn, today's children may be the first to die at a younger age than their parents.

Dr Smego's team divided 1,263 children aged between two and six years old into groups depending on their BMI, and examined their health records dating back to their birth.

The team found that children who were severely obese by the age of six began to differ from that of normal weight children at about four months of age.

BMI at six, 12 or 18 months of age ... can accurately predict children at risk for early childhood obesity Dr Allison Smego, Cincinnati Children's Hospital

Dr Smego whose findings were presented at a meeting of the Endocrine Society in Boston, US, said: 'BMI at six, 12 or 18 months of age ... can accurately predict children at risk for early childhood obesity.

'It's not currently recommended to measure BMI in children under the age of two, but we say it should be because we now know it predicts obesity risk later.

'Paediatricians can identify high-risk infants with BMI above the 85th percentile and focus additional counselling and education regarding healthy lifestyles toward the families of these children.

'Our hope in using this tool is that we can prevent obesity in early childhood.'

A study published in The Lancet journal today predicted that - if current trends continue - more than a fifth of people in the world will be obese by 2025.

That research, led by a team from Imperial College London, showed that over a 40-year period between 1975 and 2014 the global number of obese individuals had soared from 105 million to 641 million.

With each passing decade, the average person had become 1.5kg (3.3 pounds) heavier.

Not all experts are convinced by the use of BMI as a tool to measure obesity.

Children who were severely obese by the age of six began to differ from that of normal weight children at about four months of age, researchers found (file photo)

The system categorises anyone with a score above 25 as being 'overweight' while a score of over 30 classes someone as 'obese.' However, experts have criticised the technique as a 'blunt tool' because it cannot distinguish between heavy muscle tissue and fat, which is far lighter.

This means that many rugby players and well-built muscular athletes are categorised as obese even though they are extremely healthy.

Experts have been particularly critical of the tool when used to measure children, even though it is used to monitor all five and 11-year-olds as part of an NHS scheme.

Many parents of slim youngsters have criticised officials after being sent letters from school nurses telling their child to attend weight management classes.

Shirley Cramer, chief executive of the Royal Society of Public Health, has been an outspoken critic of these so-called 'fat letters'.

She said in a review last year: 'Our research finds that only one fifth of parents find the 'fat letter' useful and we believe that the letter should be seen as the beginning of a dialogue with parents, not simply flagging whether their child is obese.'

Shirley Cramer, chief executive of the Royal Society of Public Health, said last night that early interventions are ‘absolutely critical’ to tackling childhood obesity'.