The standard test to see whether people are a healthy weight does not work consistently across different ethnicities, health officials have said.
The body mass index (BMI), used to identify people's risk of developing type 2 diabetes or heart disease, might be being "applied wrongly" to people from black, Asian and other ethnic minority groups, the National Institute for Health and Care Excellence (Nice) said.
BMI is calculated using height and weight, and if someone has a score over 25 they are deemed to be overweight, and those over 30 are classed as obese. But experts said healthcare workers should apply lower thresholds to people from some ethnic groups.
Nice said doctors and nurses need to be made aware that people from black, Asian and other minority ethnic groups are at an increased risk of chronic health conditions at a lower BMI than the white population.
Professor Mike Kelly, director of the Centre for Public Health at Nice, said: "The point at which the level of body fat becomes risky to health varies between ethnic groups. Healthcare workers should apply lower thresholds to people from black, Asian and other minority ethnic groups than to those of white European descent.
"Excess body fat contributes to more than half of cases of type 2 diabetes, one in five of heart disease and between 8% and 42% of certain cancers. The number of people affected by these health conditions is far greater among black, Asian and other minority ethnic groups - despite rates of obesity among these groups being similar to the white population.
"Evidence from research into diabetes suggests a body mass index of 23, not 25, should be used in treating diabetes for people from minority ethnic groups. More research and evidence is needed to help give a figure for other weight-related diseases and the BMI figure could be even lower."