They claim the discovery, published in the journal Science, is the most clear genetic link yet to obesity in the general population.
In the UK obesity is a growing problem, with government statistics predicting that there will be 12 million adults and one million children obese in 2010 unless steps are taken to halt this increase.
Scientists from the Peninsula Medical School, Exeter, and the University of Oxford discovered a strong association between people's body mass index (BMI) and a variation, or 'allele', of the gene FTO.
They then tested this discovery in 37,000 samples of this gene from a number of regions of the UK and Finland.
People with one copy of the FTO allele had a 30 per cent increased risk of being obese compared to a person with no copies. Those with two copies had a 70 per cent increased risk, being on average three kg heavier than those with none.
Among white Europeans, approximately one in six people carry both copies of the FTO allele.
Scientists have yet to work out why FTO increases BMI and obesity.
"As a nation, we are eating more but doing less exercise, and so the average weight is increasing, but within the population some people seem to put on more weight than others," said Professor Andrew Hattersley from the Peninsula Medical School.
"Our findings suggest a possible answer to someone who might ask 'I eat the same and do as much exercise as my friend next door, so why am I fatter?' There is clearly a component to obesity that is genetic."
Professor Mark McCarthy from the University of Oxford said the study's findings are a "source of great excitement".
"By identifying this genetic link, it should be possible to improve our understanding of why some people are more obese, with all the associated implications such as increased risk of diabetes and heart disease," he added.
"New scientific insights will hopefully pave the way for us to explore novel ways of treating this condition."
Dr Mark Walport, director of the Wellcome Trust, said the study is an "exciting piece of work…[which] will open up a wealth of new avenues to understand and treat common diseases".
Click here to run an energy price comparison, and see if you could be paying less for your gas and electricity.