The finding is based on a combined analysis of 18 studies dealing with the possible dangers of second-hand smoke.

Writing in the New England Journal of Medicine, the researchers - lead by Dr Jiang He of Tulane University, New Orleans - concluded that the increased risk of non-smokers developing heart disease was small.

But they warned that because of the widespread use of cigarettes, "the public health consequences of passive smoking with regard to coronary heart disease may be important".

The researchers weighed the strengths and weaknesses of 18 studies conducted in the United States, Japan, England, China, New Zealand, Australia, Italy and Scotland.

They concluded that the risk of heart disease for non-smokers who were near smokers was 25% higher than those who were not.

They also concluded the more people were exposed to second-hand smoke, the greater the risk.

The risk was 23% greater among the people exposed to the smoke of one to 19 cigarettes per day.

For those who were exposed to the smoke of 20 or more cigarettes a day, the risk rose to 31%

"Many children are regularly exposed to cigarette smoke at home or in other environments, such as child-care facilities and schools," the researchers concluded.

"The only safe way to protect non-smokers from exposure to cigarette smoke is to eliminate this health hazard from public places and workplaces, as well as from home."

Study criticised

However, in an editorial for the journal, Dr John Bailar, of the University of Chicago, warned that the technique of pooling data from separate studies, known as meta-analysis, is "often not very reliable".

One reason is that research studies often vary widely in quality, he said. Researchers also only tended to report studies that show a positive effect.

Dr Bailar said other studies had shown smokers had a 75% increased risk of heart disease. He questioned whether the risk from second hand smoke could possibly be as high as 25%.

He said: "I find it hard to understand how environmental tobacco smoke, which is far more dilute than actively inhaled smoke, could have an effect that is such a large fraction of the extra risk of heart disease among smokers."

Dr Bailar said it was more likely that second-hand smoke posed only about two per cent of the risk to a smoker.

Clive Bates said the blood was very sensitive to smoke

Clive Bates Director of ASH, the anti-tobacco pressure group, said the study findings were consistent with the conclusions of earlier research.

He said:: "The blood appears to be very sensitive to low doses of tobacco smoke, and this means that passive smokers can face as much as one-third of the heart disease risk of active smokers.

"This is an incredibly high risk to inflict on other non-smokers, and if these figures are correct, it could account for over 10,000 cases of heart disease in the UK each year in non-smokers.