Experts believe declining numbers of female smokers over the past few decades may have had a direct impact on rising weight problems as fewer people suppress their appetites with cigarettes.

And piling on the pounds can lead to health problems including diabetes, heart attacks and strokes.

The authors, led by Dr Laurence Gruer from NHS Health Scotland, reviewed the cases of 3,613 women who never smoked.

The group, recruited in Scotland between 1972 and 1976 when aged 45-64, was monitored for 28 years.

The results showed those who never touched tobacco were more likely to be overweight or obese than their fellow smokers - 60% against 40% - with the extra weight acting as an "important contributor" to premature death.

The highest rate of obesity among non-smokers was found in low income groups, according to the study published in the British Medical Journal. Almost 70% of women in this category were overweight or obese.

Dr Gruer said: "You can certainly assume that if you are obese, you are more likely to die of things like diabetes, heart attacks and strokes."

However, his study - which also considered mortality rates in different sectors of society - found non-smokers who lived a healthy lifestyle were no more likely to die early if they were from a lower occupational group.

Dr Gruer added: "If you never smoke and you keep your weight within the reasonable limit then even if you earn below average income and live in a more disadvantaged area, you can still expect to live a long and healthy life.

"You are not doomed to die early just because you happen not to have a high income or good job or live in a leafy suburb.

"It goes against the idea that if you live in a poor neighbourhood or came from a working class background, then your health will be worse, regardless."