More than one in three adults may be misclassified by BMI - the standard test used by doctors to decide if people are a healthy weight - leaving millions with the wrong picture of their health.
The score is calculated using a simple formula based on height and weight - but new research suggests it may be far less reliable than many assume.
The findings add to mounting evidence that BMI is not fit for purpose - and can give a misleading picture of a person's true body fat and risks.
Researchers at the University of Modena and Reggio Emilia, the University of Verona and Beirut University compared BMI classifications with dual-energy X-ray absorptiometry (DXA) - the gold-standard method for measuring body fat.
The study, published in the journal Nutrients and due to be presented at this year's European Congress on Obesity in Istanbul, analysed 1,351 adults aged between 18 and 98, most of whom were women and all of White European background.
Based on BMI, 1.4 per cent were underweight, 58 per cent a healthy weight, 26 per cent overweight and 14 per cent obese - figures broadly in line with population estimates in Italy's Veneto region.
But DXA scans told a different story.
More than a third of people classed as obese by BMI were actually only overweight.
According to the most recent health survey data for England, around 64 per cent of adults are either overweight or living with obesity, based on BMI
Under the BMI system, a score of 18.5 to 25 is healthy. A score of 25 to 29 counts as overweight, and 30-plus means a person is obese, the stage at which chances of illness rocket
Among those labelled overweight, more than half were misclassified - most were in fact a healthy weight, while a smaller proportion were actually obese.
Even among those deemed a healthy weight, BMI got it wrong in around one in five cases.
The biggest discrepancy was seen in people classed as underweight - two-thirds of whom were found to be a healthy weight based on body fat measurements.
Overall, DXA suggested slightly lower rates of excess weight, with 23 per cent overweight and 13 per cent obese.
Professor Marwan El Ghoch, who led the study, said: 'Over a third of adults are misclassified when relying on BMI, which overestimates underweight, overweight and obesity compared with actual body fat measurements.'
Professor Chiara Milanese, co-author, added: 'Even when BMI and DXA find similar overall rates, the people identified are not always the same. This shows BMI can misrepresent weight in adults of different ages and genders.'
One key limitation of BMI is that it cannot distinguish between fat and muscle - meaning someone with high muscle mass may be labelled overweight, while another person with higher body fat but less muscle could fall within the 'healthy' range.
The researchers say BMI should be used alongside other measures, such as waist-to-height ratio or skinfold tests, to give a more accurate picture of health.
They also called for further studies to assess whether similar misclassification occurs in more diverse populations worldwide.
Their findings echo calls from 58 international experts for a broader definition of obesity that includes measures such as waist circumference and fat distribution, rather than relying on BMI alone.
Separate analysis of more than 300,000 adults in the US suggests that adopting these additional measures could increase the number of people classified as obese by almost 60 per cent.
Applied to the UK, that would raise the number from around 13 million to nearly 21 million people.
According to the latest Health Survey for England, around 64 per cent of adults are classed as overweight or obese based on BMI alone - a figure that underpins national policy and NHS guidance.
Obesity is linked to serious health risks including type 2 diabetes, heart disease and breathing problems, and costs the NHS more than £11 billion a year alongside wider losses from reduced productivity and welfare spending.
Researchers say the message is clear: even when standard markers of metabolic health appear normal, carrying excess body fat is linked to a significantly higher risk of serious disease.
Maintaining a healthy weight remains one of the most important steps for protecting long-term heart, liver and kidney health, they added.