A team from Oxford's Genetics of Energy Metabolism (GEM) consortium and Oxford University have picked up a stretch of rogue DNA that is carried by more than a third of the population and increases the chances of becoming fat.

The research involved 77 institutions in six countries which analysed the DNA of 90,000 individuals from the UK, USA, France, Germany, Italy, Finland and Sweden.

It revealed that the effects of the rogue DNA are exacerbated by the presence of the FTO gene, discovered last year, which was the first widespread gene known to affect obesity.

They believe their discovery could explain why some people seem to effortlessly lose weight while others have a constant battle.

The researchers say it is important people understand there is a genetic predisposition to obesity and as genetic sequences come in pairs, the effect is greatest in those with two flawed copies.

They say this may help people adopt a less prejudicial attitude towards the obese.

The scientists say unraveling the genetics of obesity could also lead to treatments for the condition; around 37 per cent carry one copy and are 12 per cent more likely to be obese.

Those with two copies of the newly-discovered DNA and two flawed copies of FTO are often more than half a stone heavier than those without such an inheritance.

The researchers suspect that the flaw affects the ability of another gene to regulate appetite and energy expenditure.

Lead author Dr Ruth Loos, from the Medical Research Council Epidemiology Unit says they were able to to assemble a large enough sample collection to allow this finding to be made, by working together with many international groups and the collaboration has uncovered more common variants that affect more people.

Dr Inws Barroso, Investigator at the Wellcome Trust Sanger Institute and one of the senior authors on the study, says the research is an excellent example of how cooperation can bring about new findings that can be missed when researchers work in isolation.

The researchers emphasize that, although gene variants can affect weight, body mass index and obesity, they are only part of the story: lifestyle actions such as good diet and regular exercise remain vital in controlling weight.

The research is published in the journal Nature Genetics.