Being "fat but fit" is a myth, doctors have warned.

People who are overweight are at increased risk of having a heart attack, no matter how healthy they are.

A major study of more than half a million people across Europe found those who were overweight or obese had up to a 28 per cent higher risk of developing coronary heart disease - where arteries in the heart become clogged leading to heart attacks or heart failure.

Overweight people were at higher risk even if they had healthy blood pressure, blood sugar and cholesterol levels.

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The latest findings contradict previous smaller studies which suggested some overweight people could be healthy if they suffered none of the negative effects usually connected to weight gain, such as high blood pressure.

Researchers, led by Imperial College London and Cambridge University, said the study showed the concept of being fat but fit was 'a myth".

Lead author, Dr Camille Lassale,said: "Our findings challenge the concept of the 'healthy obese'.

"The research shows that those overweight individuals who appear to be otherwise healthy are still at increased risk of heart disease.

"Our findings suggest that if a patient is overweight or obese, all efforts should be made to help them get back to a healthy weight. Even if their blood pressure, blood sugar and cholesterol appear within the normal range, excess weight is still a risk factor."

For the study - the largest of its kind to date - scientists looked at health data from the European Prospective Investigation into Cancer and Nutrition (EPIC) on 520,000 people from 10 countries, including the UK.

In a follow-up period of around 12 years, a total of 7,637 people in the study experienced a serious event linked to heart disease, such as a heart attack or death.

Researchers compared this group with 10,000 others in the study and analysed the link between having a heart disease event, weight and general health.

They found that healthy people who were overweight had a 26 per cent increased risk of developing heart disease compared to healthy people of normal weight. The risk for healthy obese people - with a BMI over 30 - was 28 per cent.

Scientists said this may be because being overweight eventually leads to health problems, so the people with excess weight who were healthy at the start of the study may have gone on to develop problems which led to heart disease.

Dr Ioanna Tzoulaki, from Imperial's School of Public Health, said: 'I think there is no longer this concept of healthy obese. If anything, our study shows that people with excess weight who might be classed as 'healthy' haven't yet developed an unhealthy metabolic profile.

"That comes later in the timeline, then they have an event, such as a heart attack."

Researchers also found being unhealthy was a key risk for developing heart disease, no matter what their BMI was.

Participants in the study were classed as "unhealthy" if they had three key markers, including high blood sugar, blood pressure, cholesterol or a large waist size - more than 37' (94cm) for men or 31' (80cm) for women.

These "unhealthy" people were at more than double the risk of developing heart disease, regardless of whether they were normal weight, overweight or obese.

But unhealthy obese people were at the highest risk - two and half times that of healthy people of a normal weight.

Scientists took into account lifestyle factors such as smoking, diet, exercise and socioeconomic status. They are now looking into exactly how obesity leads to heart attacks.

The study is published in the European Heart Journal.

Professor Metin Avkiran, associate medical director at the British Heart Foundation, which part-funded the research, said: "This study provides robust evidence that there is no such thing as 'healthy obesity'.

"It conclusively shows that being obese increases a person's risk of developing heart disease, even if they are otherwise healthy.

"The take-home message here is that maintaining a healthy body weight is a key step towards maintaining a healthy heart."

Earlier this year, a Dutch study of 5,300 people suggested that obese over-55s could stay healthy if they kept active for at least four hours a day.

They found exercise reduced the risk of suffering a heart attack or stroke, regardless of someone's BMI - implying it was possible to be both fat and fit.