A multimillion dollar campaign by the tobacco industry to blur the risks of passive smoking by discrediting key evidence is revealed today.

Papers unearthed from among 32 million pages of tobacco-industry documents published as part of a US court settlement in 1998 show the lengths to which the industry went to undermine a passive- smoking study by the International Agency for Research on Cancer (IARC), which reported in October 1998. It found a 16 per cent increase in lung cancer risk for non-smokers exposed to other people's smoke, which was consistent with earlier findings.

But the number of people involved in the study was too small to reach statistical significance. Although supportive of earlier studies, the finding was not by itself conclusive.

The Philip Morris company co-ordinated efforts by six leading world tobacco manufacturers to counter the study soon after it started in 1988. They included the British companies Rothmans, Imperial and British American Tobacco. The companies set up a task force, advised by the public-relations company Burson Marsteller, to "stimulate controversy" about the study. They infiltrated the research team to discover what its likely conclusions would be, commissioned their own studies and leaked stories to the media.

In 1994 Philip Morris planned to spend $2m on the campaign and allocated up to $4m on research. The IARC study cost $2m over 10 years.

Six months before the IARC published its findings, British American Tobacco leaked a story to The Sunday Telegraph saying the results were "consistent with there being no additional risk ... and could be consistent with passive smoke having a protective effect against lung cancer".

This was based on the fact that the study lacked statistical significance. Although the findings showed an overall 16 per cent increased risk - not apparently revealed to The Sunday Telegraph - because of the study's small size, the true figure could range from a 45 per cent increased risk to below zero per cent (meaning passive smoking could be protective).

Despite World Health Organisation and IARC press releases that said the interpretation of the findings in the leaked story was "false and misleading", the claims spread. In Bangladesh, BAT sent the article to a newspaper that refused cigarette advertisements.

Details of the Philip Morris campaign are given in The Lancet today, which says the "dirty war of misinformation" was likely to continue. "All policy-makers must be vigilant to the possibility of research data being manipulated by corporate bodies and of scientific colleagues being seduced by the material charms of industry. Trust is no defence against an aggressively deceptive corporate sector," it says.

The documents gathered by Elisa Ong and Stanton Glantz of the University of California show Philip Morris feared the IARC study would lead to increased restrictions on smoking in Europe. A 1993 Philip Morris memo stated its aims:

"Delay the progress and/or release of the study;

Affect the wording of its conclusions and official statement of results;

Neutralise possible negative results of the study, particularly as a regulatory tool;

Counteract the potential impact of the study on government policy, public opinion, and actions by private employers and proprietors."

The findings of the IARC study were published in the Journal of the National Cancer Institute. An editorial concluded that, with previous evidence, they presented "an inescapable scientific conclusion ... that environmental tobacco smoke is a low-level lung carcinogen."