A quarter of the UK population has an obese Body Mass Index (BMI), but a high number "deny" how severely their weight could be affecting their health, according to new figures.
Only 6% of people believe their weight problem is severe enough to be described as obese, Slimming World's annual survey showed.
Three quarters of people with an obese BMI underestimate their weight category according to the National Slimming Survey, which had 2,065 respondents.
More than one in three who are regarded as overweight said they felt weight "is the most important issue in life".
Half of those classified as obese said their weight made them feel embarrassed, while others said they felt awkward, disgusted,
ashamed, clumsy or trapped.
Dr Jacquie Lavin, head of nutrition and research at Slimming World, said: "This worrying new data reveals the complex psychological issues associated with being overweight.
"Many people, including many health professionals, believe that managing weight is just about energy balance, and that people simply need to 'eat less and exercise more'."
However, that approach can never work while so many people deny how severely their weight could be affecting their health by increasing their risk of diabetes, heart disease and stroke or while they struggle with the emotional burden of being overweight, which can affect their confidence in their ability to make healthy lifestyle changes.
"As individuals, we need support to tackle the deep-rooted psychological issues around how we feel about our weight before we can begin to make those changes."
Father-of-four Michael Druker lost 14 stone with the help of Slimming World after realising his gradual weight gain was affecting his quality of life.
He said: "I didn't really notice when I first started to become overweight. I'm quite tall and so it didn't show at first.
"Then when it did show I just felt helpless, like there wasn't anything I could do about it. So I buried my head in the sand and pretended nothing was wrong.
"Eventually, though, I ended up with this long list of things that I couldn't do because of my weight and I knew I had to do something about it.
"As a man, I thought slimming clubs were for women and that I'd feel humiliated by going to one but nothing could have been further from the truth. Everyone made me feel welcome and it didn't matter at all to them that I was a man, they just saw me as someone, like them, who needed support to lose weight.
"With their help I gained the confidence to really get to know myself and to work through my issues around food, finding new ways of eating and becoming more active that fit with my lifestyle."