Researchers from University College London (UCL) who were funded by the Biotechnology and Biological Sciences Research Council created a new experiment to see if eating when full was linked to a gene called FTO**. To do this, they observed the behaviour of 131 four to five year olds who were offered a plate of biscuits after they had eaten a meal.

They found the children who ate more biscuits were more likely to have one or two of the 'higher' risk versions of the FTO gene. This research could help pave the way to a better understanding of the processes that lead children to become overweight or obese - one of the biggest risk factors for cancer.

Lead author, Professor Jane Wardle, director of Cancer Research UK's Health Behaviour Research Centre at UCL, said: "Previous research has shown that the FTO gene is linked to larger body size. We believe this research published today tells us more about how some children are more responsive to signals in their bodies encouraging them to eat when full than others. Knowing how the genes work is the first step to minimising these negative effects."

"We hope this research will help improve our understanding of the causes of childhood obesity so that better measures can be taken to reduce it. Children with higher risk versions of the gene might be helped if parents do their bit to keep temptations out of the home."

Professor Wardle continued: "The occasional treat won't do us any harm - but this study showed that some children don't know when to stop - which could lead to the onset of obesity and a lifetime of health problems. We know the best way to maintain a healthy body weight is to eat a diet with lots of fibre, vegetables and fruit as well as keeping portion sizes down, and being physically active."

Research has shown that obesity increases the risk of cancer of the bowel, womb, kidney and post - menopausal breast cancer. In the UK alone, it is estimated that 13,000 people every year could avoid cancer by maintaining a healthy body weight. Obesity has also been linked to increased risk of other cancers including cancer of the gallbladder, oesophagus and pancreas, but more research is needed to confirm this.

In the study published today, the researchers also looked for a genetic connection between the FTO gene and children's interest in taking exercise, and didn't find a link.

Sara Hiom, director of health information at Cancer Research UK said: "A genetic propensity to overeating doesn't doom a child to a lifetime of obesity. But it does allow us to think about how we can best help the children most at risk of becoming obese. An important part of this is to urge parents to provide healthy snack options such as carrot sticks rather than chocolate biscuits and ideally to encourage children to stop eating when full. We calculate that a quarter of all cancer deaths are caused by unhealthy diets and obesity.

"This work attempts to understand what underlying biological processes are involved in eating too much. But it's important to remember that not all children with these 'high' risk genes will over eat - other influences are very important too - including the eating habits of parents and the types of food made available."

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