A working group from the International Agency for Research on Cancer (IARC), which is part of the World Health Organization, examined all of the major studies looking at smoking and cancer.

After a five-day meeting in Lyons, France, this week, they suggested non-smokers who are exposed to second-hand smoke are between 20% and 30% more likely to develop lung cancer.

Passive smoking is quite clearly more than just the nuisance many of the world's tobacco companies would have us believe

Marsha Williams, ASH

The experts also found cancers of the stomach, liver, uterus, cervix, kidney and myeloid leukaemia could be caused in part by smoking.

The group of 29 experts from 12 countries found second-hand tobacco smoke was carcinogenic to humans and that typical levels of passive exposure have been shown to cause lung cancer among people who have never smoked.

This means hundreds of thousands of deaths worldwide from these cancers could now be linked to smoking.

Definite link

Dr Kurt Straif, who organised the IARC meeting, said the group examined 50 studies examining the link between passive smoking and cancer.

Speaking to BBC News Online, he added: "This is the first time that a global organisation has concluded that exposure to second hand smoke is linked to cancer."

One of the experts, Sir Richard Doll, said: "Environmental tobacco smoke that people experience at work or at home is definitely a cause of lung cancer.

"That has been discussed for a long time but this is the first time a group of independent scientists have reviewed all the evidence and said there is no question it is a cause of lung cancer."

Sir Richard said the findings should have a significant influence on health policies around the world and could strengthen arguments for a ban in this country on smoking in workplaces.

The experts also stated that one half of all persistent cigarette smokers are eventually killed by a tobacco-related disease.

Half of these deaths occur in middle age, which means they lose an average of 20-25 years of non-smoker life expectancy.

Annually tobacco accounts for millions of cancer deaths around the world, and it is the largest cause of preventable cancers.

However it causes a greater number of premature deaths from cardiovascular and lung diseases and strokes than from cancer.

Apart from cigarettes, other forms of tobacco smoking such as cigars and pipes increase the risks for cancer of the lung, head and neck.

Mounting evidence

Marsha Williams, of the anti-tobacco campaigning group ASH, said the findings added to the mounting body of evidence of the health risks from second-hand smoke.

She said: "Passive smoking is quite clearly more than just the nuisance many of the world's tobacco companies would have us believe.

"People are harmed and killed by it and it is time industry, government and smokers themselves woke up to this fact."