Scientists have discovered the first clear genetic link to obesity that is carried by significant numbers of people, according to a study yesterday. One copy of the gene leads to a 1.2kg (2.6lb) weight increase while those with two copies are on average 3kg heavier.

The scientists say it will open up new avenues of research into who is susceptible to obesity and how best to prevent the condition that is the second largest cause of death in the UK.

The focus for treating obesity has been on changing lifestyle through diet and exercise, but the research shows genetic factors can make a big difference. "We live in a blame culture in which when people gain weight it is entirely their fault," said Graham Hitman at Queen Mary's school of medicine and dentistry, London University, one of the authors of the study. "Improving lifestyle is still the key to reducing the obesity epidemic but some people will find it harder because of their genes."

The finding is significant because the changed version of the gene is relatively common, at least in the European populations studied so far. The researchers report in the journal Science that around half of people have one copy of the offending version while 16% have two copies. Those with two copies are 67% more likely to be obese - defined as a body mass index of 30 or more.

Adult obesity rates have nearly quadrupled in the UK over the last 25 years, and two-thirds of adults are overweight. Of these, nearly a quarter are obese. In England, more than 30,000 deaths a year are caused by obesity, the second-largest cause after smoking-related illness.

The research team predicts that other genetic causes will emerge. "This is still only one player out of many," said Mark McCarthy at the Wellcome Trust centre for human genetics at Oxford University, who led the study. "[Three kilogrammes is] a lot in some respects, but that doesn't explain why some people are 50kg heavier than others."

The team was first alerted to the involvement of the FTO gene in a study looking for genetic factors involved in type 2 diabetes.

"We were able to show that it is associated with differences in weight and that weight is due to differences in fat," said Professor McCarthy.

The team does not know what the gene does, but Prof McCarthy said that was one thing that made the discovery particularly exciting. "What we are really looking for are novel clues to how weight is regulated and how obesity comes about. If we understand how this gene is translating into differences in weight and differences in obesity then we might be able to use that to understand, for example, why treatment A is better in some people than treatment B."

Although it might be tempting to add "it's my genes' fault" to "I'm big boned" as the excuse for being overweight, the discovery does not change advice to patients. "Whether you have this risk factor or not, if you are overweight you should eat less and exercise more," said Prof McCarthy.

"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 co-author Andrew Hattersley from the Peninsula Medical School in Exeter. "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?'"

According to figures from the National Audit Office, being obese can take up to nine years off a person's lifespan. It makes them more likely to develop a range of health-related problems such as type 2 diabetes, heart disease, stroke, osteoarthritis, high blood pressure, gallstones, infertility and depression.

FAQ Fat factors

Why is this discovery so important?

Obesity is more complicated than "calories in from food minus calories out by exercise". One study which repeated Morgan Spurlock's Supersize Me documentary found that the fast food diet hardly touched some people while others ballooned by 15% in just two weeks.

The new study is significant because the effect is so big and the genetic change is carried by so many people. "This represents the first clear example of a common genetic variant," said Steve O'Rahilly, professor of clinical biochemistry and medicine at Cambridge University.