Three quarters of obese people were in denial over their weight problem and most thought their diet was healthy, researchers found.
The National Slimming Survey 2009, which questioned more than 2,000 people, found the perceptions of being overweight had changed as the population's waistlines ballooned.
It showed 7 per cent of those polled believed they were obese - but the true figure was 27 per cent, according to measurements given by the respondents.
The research, conducted by Slimming World and YouGov, suggested millions of Britons could be unaware of the health risks associated with putting on weight.
Dr Jacquie Lavin, head of nutrition at Slimming World, said: "People in the UK are getting heavier and that is likely to be having an effect on their perceptions of what qualifies as obese.
"For years experts have agreed that losing just 10 per cent body weight can lead to significant improvements in health - but if people don't even realise they have a serious weight problem it can be difficult to address."
The study found being overweight had a "devastating impact on people's health and happiness", Dr Lavin said.
Morbidly obese people - with a body mass index (BMI) of more than 40 - were 10 times more likely to describe their health as "very poor". They were more prone to conditions such as high blood pressure, diabetes, arthritis and asthma, as well as depression and low self-esteem.
Experts say a healthy BMI, calculated using height and weight, is between 18.5 and 25. Those with a BMI greater than 30 are obese, according to the method.
The study showed 52 per cent of morbidly obese people thought their diet was healthy.
Having children was the top reason given by women for gaining weight (22 per cent), while most men blamed moving to a less active job (27 per cent).
It also found 38 per cent of overweight people had never tried to lose weight.