Millions of people in the UK could be failing to recognise that they are obese - putting themselves at risk of early death, a new report suggests.
There is poor understanding of obesity-related health risks and many do not even recognise that they are obese, Nuffield Health said.
New research conducted by the charity found that two-fifths of obese people had no concerns about serious illness due to their weight.
Being obese increases the risk of a number of potentially fatal diseases such as heart disease, some types of cancer, stroke and type 2 diabetes. But many people simply believe they are overweight instead of clinically obese, a charity spokeswoman said.
The charity questioned 3,100 people about their health and well-being - including about perceptions of their weight.
Half believed themselves to be overweight and 6% thought they were obese. But body mass index (BMI) tests revealed that 17% were clinically obese - classed as such when they have a BMI score over 30 - and 3% measured as "morbidly obese", with a BMI of 40 or over.
The charity said that there is a "clear misconception" over what weight is considered to be obese.
If the figures are extrapolated, this means around six million people in the UK could be at risk by not recognising that they are obese and the associated health risks, a charity spokeswoman said.
Meanwhile, 44% of those who are obese said they have no concerns about serious illness due to their weight. And many did not recognise the links between obesity and health problems.
Eight in 10 of those polled did not know about the link between obesity and some cancers such as breast and bowel cancer.
Around two-thirds were unaware about obese people's increase risk of liver disease or osteoarthritis. And 46% were not aware about the link between obesity and stroke.
Dr Davina Deniszczyc, medical director for well-being at Nuffield Health, said: "There is a very big difference between being slightly overweight and clinical obesity. Once BMI reaches 30, the body experiences physiological changes which can put massive pressure on the vital organs - increasing the risk of numerous conditions including heart attack, stroke and liver disease.
"We are seeing a vast number of people unwittingly straying into dangerous medical territory and perhaps not realising that the obesity awareness campaigns are directed at them.
"As healthcare professionals, we need to prioritise the health of our patients over the risk of hurt feelings caused by a frank and open conversation about their weight.
"It's vital that people have the information they need in order to make informed decisions about their health.
"While we need to increase access to clinically recognised weight management programmes and treatments, some people may simply benefit from structured information and goal-setting, an exercise regime or through tackling the root cause of their obesity with the help of a support group or a weight management specialist; currently, these conversations are not taking place."