The idea of "healthy obesity" is a myth, a study of 3.5 million Britons suggests.
Having excess fat increases the risk of suffering heart disease by half even when blood pressure and cholesterol levels are normal, according to the research.
People regarded as obese but healthy also have an increased risk of stroke and almost double the risk of heart failure, researchers found.
Experts have long debated whether people can truly be "healthy obese" or "fat but fit", characterised by having normal markers of metabolic health despite having a body mass index (BMI) of 30 or more.
While most obese people have an increased risk of developing conditions such as heart disease, stroke and Type 2 diabetes compared to those of a normal weight, some seem to buck that trend and remain "healthy".
Previous research has suggested up to one in three obese people are healthy despite carrying extra fat.
A team from the Institute of Applied Health Research at the University of Birmingham has now cast doubt on this idea in the biggest study of its kind.
Their study, presented at the European Congress on Obesity in Porto, Portugal, analysed health records from 1995 to 2015 for 3.5 million adults who were initially free from heart disease.
The team looked for markers of being metabolically healthy, having normal blood pressure and cholesterol and no diabetes, while also being obese.
They then tracked how many people suffered one of four cardiovascular conditions, coronary heart disease (CHD), cerebrovascular disease (including stroke), heart failure and peripheral vascular disease (a disorder of blood circulation).
The result showed, compared to healthy people of a normal weight, those regarded as healthy and obese had a 49% increased risk of coronary heart disease.
They also had a 7% higher risk of stroke and a 96% increased risk of heart failure.
These figures held true even when factors such as whether people smoked were taken into account.
While there was an overall lower risk of peripheral vascular disease, healthy obese people who never smoked had an increased risk.
In total, there were 165,302 adverse health events during an average 50-year follow-up of patients, with 61,546 cases of CHD, 54,705 of mini stroke and stroke, and 25,254 cases of heart failure.
There were also 23,797 cases of peripheral vascular disease.
Lead author, Dr Rishi Caleyachetty, told the Press Association: "People should seek the help of health professionals to achieve their optimum weight.
"The idea of being healthily obese is a myth and there is much that health professionals and people who are obese can do to reduce their risk of disease."
He added: "What we have shown in this study of 3.5 million people is that metabolically healthy obese individuals, compared to those of normal weight, are at higher risk of conditions such as heart disease, stroke and heart failure.
"Metabolically healthy obesity is not a harmless condition, and it would be incorrect to think so.
"It's actually better not to use this term as it can create a lot of confusion."
Tam Fry, chairman of the National Obesity Forum, said: "Can you be fat and fit? Ask any group of rugby forwards who shift their bulk up and down a 100m field for 80 minutes and you'll get a 'yes'.
"Ask scientists stuck in a lab and you might not.
"Unfortunately, the Birmingham researchers appear not to have factored in recent genetic evidence that goes a long way to explaining why so many fat people appear not to suffer ill health.
"These people have the ability to store large amounts of fat around their bodies yet remain fit. And so the debate rages on. "
Dr Mike Knapton, associate medical director at the British Heart Foundation, said: "This is another study highlighting that, if you are overweight, you are more likely to suffer from heart disease.
"It's not often that research on this scale and magnitude is able to clarify an age-old myth.
"These findings should be taken extremely seriously and I'd urge healthcare professionals to take heed."