Millions more Britons could be labelled obese in a proposed major shake-up of the body-mass-index classification (BMI) system. 

The weight-to-height ratio is backed by the World Health Organization and used by doctors to determine if a person is at risk of a host of diseases from diabetes, heart attacks and strokes due to excess fat. 

Under the current 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 rockets.

But now Italian research suggests the model needs an overhaul for the over-40s and anyone with a BMI of 27 or more should be classed as obese. 

NHS data shows only 26 per cent of adults have a BMI of 30-plus. 

Celebrities such as Avengers star Chris Hemsworth, Australian actress Rebel Wilson, and Britain's own Tom Hardy would also technically tip over into an 'obese' BMI under the proposed change

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

More than 42million adults in the UK will be overweight or obese by 2040, according to projections by Cancer Research UK

But the average BMI of all adults over 40 in the UK varies between 27.7 and 27.6, suggesting the majority would be declared obese, and at risk, under the proposed overhaul.

It means many people could, in theory, be obese without realising it.

Celebrities like Avengers star Chris Hemsworth, Australian actress Rebel Wilson, and British A-lister Tom Hardy would also technically tip over into an 'obese' BMI under the changes.   

Presenting their findings at the European Congress on Obesity in Venice, the authors of the new study said the current BMI cut-off for obesity may not be suitable for middle-aged and older adults. 

They said 40-plus bodies have an increased fat build-up around the waist that, combined with age-related muscle decline, means there is no overall change in total weight.

This means that despite piling on the flab, the BMI system fails to raise the alarm and people don't realise they are at risk of obesity related health problems. 

Researchers backed up their findings with a study of 4,800 Italian adults, aged between 40-to-80-years, split roughly equally between men and women.

Of this group, 38 per cent and 41 per cent of women had a BMI of 30 or more, obese under the current rules.

However, when the researchers used high-tech scans to measure their actual percentage of body fat, 71 per cent of men and 64 per cent of women were assessed as obese. 

This meant that nearly half of men and women with concerning levels of fat were being falsely reassured by the BMI system.  

Comparing the findings, they found that using a BMI cut-off of 27 for obesity instead covered nearly 9 out of 10 of the obese patients detected in the body scan method. 

Author of the study, Professor Marwan El Ghoch an expert in metabolic health at the University of Modena and Reggio Emilia, said: 'This new BMI cut off recognises the physiological differences between middle-aged and older adults and younger populations.'

Professor Antonino De Lorenzo, co-author and an expert in biomedicine from the University of Rome, added: 'If we continue to use the WHO standard for obesity screening, we will miss many middle-aged and older adults who are at risk for obesity-related diseases including type 2 diabetes, heart disease, and some cancers.

'Establishing this new BMI cut-off point in clinical settings and obesity guidelines will be beneficial to the potential health of millions of older adults.'

The study, which was presented at the conference, said their work does have limitations. 

For example, it was conducted in one part of Italy so the findings may not apply to other parts of the world. 

Another was that they didn't account for other factors like diet, exercise levels, and sleep patterns, which could also influence obesity among the participants, not just age.

HOW TO CALCULATE YOUR BODY MASS INDEX - AND WHAT IT MEANS 

Body mass index (BMI) is a measure of body fat based on your weight in relation to your height. 

Standard Formula:

• BMI = (weight in pounds / (height in inches x height in inches)) x 703

Metric Formula:

• BMI = (weight in kilograms / (height in meters x height in meters))

Measurements:

• Under 18.5: Underweight
• 18.5 - 24.9: Healthy
• 25 - 29.9: Overweight
• 30 - 39.9: Obese 
• 40+: Morbidly obese 

They called for similar studies on a larger scale to be held in other parts of the world to confirm their results. 

Experts have previously criticised BMI as a measure of health.

Devised by a Belgian mathematician in the 1830s, doctors have relied on BMI for almost two centuries.

One flaw is that it is incapable of differentiating between fat distribution and muscle mass.

Realistically, this means a fit rugby player and couch potato of the exact same height and weight share the same scores — even if the former has a ripped physique and the other carries a spare tyre.

This flaw would still apply under the proposed BMI of 27 is obese shake up.

Hollywood hunk Chris Hemsworth, 40, who stars in the new Mad Max film, would technically be obese based on 6ft 3in tall actor's 15st 6lbs weigh in during his time as Thor in the Avengers series.

Fellow Australian, actress Rebel Wilson, 44, who managed to achieve her weight goal of 11st 4lbs in a dramatic weight-loss journey, would technically be obese under the shake-up.

And Tom Hardy, 46, who weighed in at 14st 2lbs for his role as the villain Bane in the Batman film The Dark Knight Rises, would also be deemed fat under the system change. 

Obesity has been well established as increasing the risk of serious health conditions that can damage the heart, such as high blood pressure, as well as cancers.

Being too fat has been estimated to cause one in 20 cancer cases in Britain, according to the Cancer Research UK. 

Britain's obesity crisis is also estimated to cost the nation nearly £100billion per year. 

This colossal figure includes the health harms on the NHS as well as secondary economic effects like lost earnings from people taking time off work due to illness and early deaths.