Children regularly exposed to smoking are three times more likely to contract lung cancer in later life than those in non-smoking homes, research suggests.

The Imperial College researchers tracked the progress of more than 123,000 participants over seven years.

They told the British Medical Journal that the link between lung cancer and passive smoking was "significant".

Health charity Cancer Research UK said the study raised a "terrifying spectre" for smoking parents.

The researchers tracked 123,479 volunteers - some of whom had never smoked, others had stopped smoking, but all had been exposed to second-hand smoke in their childhoods.

Over a seven-year period they found that 97 people developed lung cancer and 20 more had related cancers such as cancer of the larynx.

In addition, 14 died from chronic obstructive pulmonary disease.

"Environmental tobacco smoke exposure during childhood showed an association with lung cancer, particularly among those who had never smoked," the researchers said.

The team concluded that the study reinforced past research about the cancerous effects of passive smoking.

The researchers also found that ex-smokers faced up to twice the risk of respiratory diseases from passive smoke than those who had never smoked.

They believe this is because their lungs were already damaged - making them more at risk to the effects of passive smoking.

The British Medical Association (BMA) said the "important study" confirmed that passive smoking kills.

"The results show clearly that second-hand smoke causes cancer of the lung, mouth and throat," a BMA spokesman said.

Professor Robert West, Cancer Research UK's director of tobacco studies, said society's attitude towards passive smoking "has to change".

"As a society we recognise that non-smokers need to be protected from carcinogens when at work but we are not doing enough to protect the most vulnerable non-smokers of all - children," he said.

Amanda Sandford, research manager for Action on Smoking and Health (ASH), called for a smoking ban in all public places.

She added: "The best thing parents can do for the health of themselves and their children is to stop smoking."

Simon Clark, director of the smokers' lobby group Forest, said: "The effects of passive smoking are notoriously difficult to measure.

Most studies are based on imprecise recall and anecdotal evidence concerning the exact amount of exposure to environmental tobacco smoke. Yet this report, like so many, adopts a preposterous pretence of precise measurement which immediately arouses suspicion.

"To isolate the effect of environmental tobacco smoke on lung cancer cases would require an examination of all possible alternative causes.

"Unfortunately it is just another example of anti-smoking hysteria, a further attempt to demonise smokers for their habit."