A study found people with the flawed DNA ate between 120 and 290 calories a day more than others.

Millions of Britons carry a 'greediness' gene that makes them eat more than others.

People with the genetic flaw take in up to 300 more calories a day that those with normal DNA - the equivalent of a plate of chips or a large bar of chocolate.

The finding explains why those with the rogue version of the FTO gene - carried by more than half of Britons -  tend to be fatter than others.

The breakthrough could pave the way for new obesity drugs.

It could also shed light on why some people find it harder to stick to diets than others.

The researchers from  Aberdeen University and the city's Rowett Research Institute weighed and categorised all the food eaten by 150 men and women for a week.

Blood samples were tested to find out whether the volunteers carried the rogue gene.

They found that people with flawed DNA ate between 120 and 290 calories a day more than others.

The extra calories were split evenly between the different food groups - meaning they were not specifically drawn to fatty or sugary snacks. No link was found with metabolism, the journal Obesity reports.

Previous research has linked the gene to obesity and diabetes.

An estimated one in six Britons carry two rogue copies of the gene, increasing their risk of obesity by 70 per cent and diabetes by 50 per cent.  These people are on average almost half a stone heavier.

The 50 per cent or so who have inherited just one flawed FTO gene are 30 per cent more likely to be obese than those that have two normal copies of the gene and 25 per cent more likely to develop diabetes.

But, until now it wasn't clear whether the gene led to weight gain by stimulating appetite or slowing down metabolism.

Researcher Professor John Speakman said the finding could explain why diets are often doomed to failure - and stressed that people with big appetites are not necessarily greedy.

'Our data clearly suggests that people with this variant of the FTO gene may become fatter because they are driven to consume food,' he said.

'If that is the case, it just shows why attempts to control you intake is so difficult because it is being driven by a deep, fundamental genetic process.

'Greater consumption of food does not mean that people are being greedy.'

The researcher added that while genetics are only one factor involved in weight gain, details of the key genes could lead to new treatments for obesity.

Research published by Professor Speakman earlier this month concluded that over-eating, rather than lack of exercise, is to blame for rising levels of obesity.

The study found that contrary to popular belief, we are no less active than 20 years ago.

We are just as active as people in Third World countries who lack the luxuries of Western life, and weight-for-weight we use similar amounts of energy to wild animals.

The studies come as Britain fights the worst weight problem in Europe, with a quarter of women and a fifth of men so overweight that their health is at serious risk.

Children fare little better, leading to warnings that unless action is taken now, they are at risk of dying at a younger age than their parents.