The risk of developing lung cancer from passive smoking may have been overstated, scientists have found.

Research analysing the findings of 37 trials that examined the impact of passive smoking found there was an increased risk of lung cancer of nearly a quarter (24%) in people exposed to passive smoke.

But a team from Warwick University argues that the finding is unreliable, and that in reality the increased risk of lung cancer is lower.

Professor John Copas and Dr Jian Qing Shi say that research which suggests an increased risk is more likely to be published than research which does not.

Therefore, an analysis of the published literature will give a false picture of the true risk.

The Warwick team has re-analysed the 37 trials, taking account of the fact that the data is likely to be biased.

Writing in the British Medical Journal, they conclude that the increased risk of lung cancer from passive smoking is more likely to be around 15% as opposed to 24%.

They therefore suggest that previous levels of risk should be interpreted with caution.

A spokesman for the anti-smoking charity Action on Smoking and Health (ASH) dismissed the findings of the research.

He said the data still showed beyond doubt that passive smoking was a risk to health.

He said: "Whilst debates over the precise level of risk exist, the effects of passive smoking remain clear.

"All credible scientific evidence shows that passive smoking irritates in the short term and is a major cause of illness in the long term."

ASH say that passive smoking is responsible for 600 cases of lung cancer and 12,000 cases of heart disease a year.

FOREST, the smokers' rights organisation, welcomed the study.

Director Simon Clark said: "To put passive smoking in perspective, the average annual risk of non-smokers getting lung cancer is 10 per 100,000 people.

"According to anti-smoker estimates, this risk is increased by 20-30% if non-smokers are exposed, long-term, to ETS (environmental tobacco smoke). That is an additional 2 or 3 people per 100,000.

"The team from Warwick University say the risk is probably half that. In other words, one or two non-smokers in every 100,000 may be at risk from passive smoking, and only if they are repeatedly exposed to tobacco smoke over many, many years."