Lopez-Jimenez made the claim at the National Obesity Forum in London.
He explained the BMI method, which measures the proportion of a person's weight against his height, fails to distinguish between lean mass and body fat. Because of the BMI's weakness as a benchmark, U.K.'s obesity problem may be worse than thought, the doctor said.
Lopez-Jimenez said research has shown that one in five Britons with normal BMI has excess body fat, which places them at risk from metabolic ailments like diabetes, which may lead to heart diseases.
Based on the BMI measure alone, two-thirds of adult Britons are considered overweight. However, Lopez-Jimenez said based on studies done on over 2,000 adults by the Mayo Clinic, a higher 75 percent of adult Britons are obese.
In lieu of BMI, Lopez-Jimenez suggested the use of a new method called Bioelectrical Impedance Analysis to check the excess weight of their patients. It works by sending an electric current through the body, which facilitates experts to detect if it encounters fat tissues, that leads to a proper estimate of body fat percentage.
Meanwhile, in Scotland, Scottish Education Minister Fiona Hyslop announced every Scot child between 5 to 8 will be provided free lunch daily to help improve the country's diet and health. The move was made after tests in primary schools discovered pupils ate 32 percent more if their meals were free.
The health initiative is expected to cost Scotland $52.2 million (30 million pound) to implement.
