British people perceive themselves to be much slimmer around the waist than they really are, research shows.
Experts fear that by failing to recognise their increasing girth some people may be missing a key warning sign of developing type 2 diabetes.
The University of Leicester study found men under-estimated their waist size by an average of 3.1 inches (7.9cm), women by an average of 2.2 inches (5.5cm).
Up to 750,000 people in the UK have type 2 diabetes but do not know it.
Type 2 diabetes can led to devastating complications such as blindness, heart disease, stroke, amputation and kidney disease.
Eight out of ten people diagnosed with the condition are overweight at the time of diagnosis.
The researchers asked 502 people to estimate their waist size.
People from white European backgrounds were worse than people from South Asian backgrounds when estimating measurements - an average 2.9inches (7.4cm) out, compared to 1.6 inches (4.1cm) for South Asians.
Lead researcher Professor Kamlesh Khunti said people with type 2 diabetes could remain undiagnosed for up to 12 years.
He said the government had recently announced its intention to set up a screening programme to spot early signs of heart problems, kidney disease and diabetes, but self-assessment still had a key role to play.
He said: "This research shows that people need to be educated about the risk of developing type 2 diabetes and about accurately assessing their waist circumference."
Douglas Smallwood, of the charity Diabetes UK, said: "To believe that you are more than three inches slimmer than you are is to ignore a clear warning of a risk of diabetes.
"It can take years for symptoms of diabetes to emerge so simple indicators like waist size are important signals."