Being seriously overweight also hastens the onset of heart disease by an average of seven years compared to people who stay slim.

It has long been known that obesity is linked to heart disease, but researchers from Duke University Medical Center, North Carolina, have become the first to actually quantify the impact on subsequent life expectancy.

The researchers looked at 12 years of detailed data on more than 9,000 heart patients.

They found they could pinpoint how long it would be before a person developed the symptoms of heart disease, such as chest pain and shortness of breath.

They told a meeting of the American Heart Association that the average age of the patients who came to the clinic was 64.

However, the average age of the overweight patients was 61, and that of the most obese was 57.

Big difference

More than half the UK population is overweight

The researchers, led by Dr. Eric Eisenstein, also calculated the impact of the earlier onset of heart disease on life expectancy of patients with heart problems.

They concluded that normal-weight adults had a life expectancy of 78, for overweight people it was 77, and for the most obese 74.

The researchers also found that heavier patients were more likely to suffer from high blood pressure, diabetes, high blood cholesterol and a family history of heart disease.

In a statement Dr Eisenstein said: "It's a clear message. Patients who weigh more than is healthy for them are coming into the hospital earlier because of heart disease, are living with that disease longer and dying slightly earlier than patients of normal weight."

Obesity is calculated using a formula called body mass index (BMI), which divides a person's weight in kilograms by height in metres squared.

Someone 5 feet 9 inches tall who weighs 169 pounds has a BMI of 25 while at 203 pounds he or she has a BMI of 30.