The findings mean that being fat in middle-age increases the risk of dying early as much as smoking.

If a woman is obese and a smoker at 40, she risks dying 13.3 years sooner than a slim non-smoker.

An obese male smoker was found to lose 6.7 years from their life expectancy.

Dutch researchers analysed data from just under 3,500 volunteers in Framingham, Massachusetts, USA from 1948 to 1990.

They found that even if people lost weight later on in their lives, they were still at a higher risk of dying early.

Obese female non-smokers lost an average of 7.1 years and men lost 5.8.

Non-smokers who were overweight, but not obese, lost three years.

'Preventable disaster'

Obesity is defined as having a body-mass index (BMI) of 30 or above. Doctors consider a BMI of 25 or under to be healthy.

A person's BMI is calculated by dividing your weight in kilograms by the square of height in metres.

Obesity is already known to increase the risk of developing Type 2 diabetes and high blood pressure.

Dr Anna Peeters, of the Netherlands Morbidity Research Unit, who led the study, said: "We concluded that obesity in adulthood is associated with a decrease in life expectancy of about seven years, both in men and women.

"The magnitude of this loss is similar to that associated with smoking.

"The smoking epidemic in the Western world is waning.

"However, a new fear should be the increasing prevalence of overweight and obesity in young adults, which heralds another potentially preventable public health disaster."

She added: "This time, we must pay attention earlier and firmly establish research for more effective prevention and treatment as top priorities in public health."

Higher risk

Dr Serge Jabbour, director of the weight-loss clinic at Thomas Jefferson University in Philadelphia said: "The message is that you have to work on your weight.

"If you wait a long time, the damage may have been done."

He added: "This study is saying that if you are overweight by your mid-30s to mid-40s, even if you lose some weight later on, you still carry a higher risk of dying."

The research is published in the journal Annals of Internal Medicine.

Source: BBC News.