An anti-smoking pressure group accused the British Medical Journal of recklessness yesterday over its decision to publish research funded by the tobacco industry showing passive smoking was less harmful than has been thought.

The 40 year study of 118,000 Californians concluded that breathing other people's tobacco smoke had no impact on deaths from heart disease and lung cancer and only a small effect on the level of respiratory disease.

The findings challenge the conventional wisdom, established by a series of research studies, that passive smoking increases the risks of heart disease and lung cancer by about 25 per cent. The harmful effects of environmental tobacco smoke have driven the trend for smoke-free offices and public spaces over the past decade.

The study was also criticised by the British Medical Association, the owner of the BMJ. Vivienne Nathanson, head of science and ethics, described it as flawed, and said most of the data had been around for decades and was judged by many expert groups to be inadequate to measure passive smoking accurately.

"There is overwhelming evidence, built up over decades, that passive smoking causes lung cancer and heart disease, as well as triggering asthma attacks," she said

The tobacco industry has long sought to discredit research showing that passive smoking is harmful. It made extensive use of a flawed World Health Organisation report published in 1998 that was interpreted to suggest passive smoking could even protect against disease.

Amanda Sandford of Ash, the anti-smoking pressure group, said: "This paper is just the latest in a long campaign to sow the seeds of doubt about the dangers of breathing in environmental tobacco smoke. The authors appear to be deliberately downplaying the findings to suit their tobacco industry paymasters." She added: "Questions will inevitably be asked about the decision to publish research conducted by scientists in the pay of the tobacco industry. This could be very damaging as it will be used by industry lobbyists to argue against laws to ban smoking in public places and workplaces."

The researchers, James Enstrom of the University of California, Los Angeles, and Geoffrey Kabat of the State University of New York, acknowledge in a note at the end of their paper that they received funding from the Centre of Indoor Air Research, which was largely supported by American tobacco companies. They describe themselves as "lifelong non-smokers whose primary interest is an accurate determination of the health effects of tobacco".

Professor Martin Jarvis, an expert on smoking research at the University of London, defended the BMJ's decision to publish the paper. "Historically, there have been many instances of research funded by the tobacco industry which were not worth the paper they were written on. But that does not mean this paper is not worth looking at. Science is science, and one must not take the view that anything which has got any association with the industry is wrong."

But he warned against making too much of the passive smoking paper's conclusions. "There is a huge literature on the health effects of passive smoking, and this is one study. You have to put it in the context of everything else."

In a statement, the BMJ said yesterday: "Papers published by the BMJ group reflect developments in the field of scientific research, and do not represent the editorial board's personal views. The decision to publish a paper is only taken after careful consideration and following a strict submission process which includes peer review. It is inevitable that some research may at times be regarded as controversial."