Rates of obesity are on the rise
More Britons are failing to recognise they are overweight, research shows
A comparison of Great Britain household surveys from 1999 and 2007 show the number of people classed as clinically overweight or obese has increased.
But fewer adults now correctly class themselves as overweight, researchers report in the British Medical Journal.
One expert said the results were "disappointing" in light of greater awareness of the problems of obesity among the general public.
In each of the surveys adults were asked to give their height and weight which was used to calculate their BMI.
And they were asked which clinical weight category - very underweight, underweight, about right, overweight or very overweight - they thought they fell into.
The proportion of obese people had nearly doubled from 11% in 1999 to 19% in 2007.
In 1999, 43% of the population had a BMI that put them in the overweight or obese range, of whom 81% correctly identified themselves as overweight.
But in 2007, 53% of the population had a BMI in the overweight or obese range, but only 75% of these correctly classed themselves as overweight.
The researchers from the Health Behaviour Research Centre at University College London said one reason for the findings could be that as a greater proportion of the population becomes overweight, people's perception of what is "normal" changes.
Study leader, Professor Jane Wardle, said: "The other explanation we put forward was that the media often illustrate articles about overweight with a person with a very high BMI giving the impression that is the size that's important.
"Half of those with a BMI in the 25 to 30 range did not recognise they were overweight and that's the range we'd like people to start taking action so their weight doesn't get any higher," she said.
One upside to the findings was that women who are a healthy weight are now less likely to believe they are overweight, which had been a concern in the past, she added.
Dr Ian Campbell, a GP and medical director of Weight Concern said: "Despite a much greater awareness among the public about the problems of obesity it seems fewer are recognising the problem in themselves.
"I agree this may be due to changing social norms - ie, as more of us become obese, the average weight and average appearance, has become heavier and rounder."
He added people needed to be made aware of how they measure themselves.
"Not just with scales as this seems to be easily misunderstood, but with specific waist measurements or body fat analysis," he said.