The risks of passive smoking could be twice as bad as previously feared, the British Medical Journal
The full effects of passive smoking may have been underestimated
Researchers from London's St George's Medical School and the Royal Free hospital found passive smoking increased the risk of coronary heart disease by 50-60%.
The team, which studied 4,792 men over 20 years, said earlier studies which had found a 25-30% increased risk focused on people living with smokers.
They did not take account of exposure at work and other places, it added.
Doctors at the British Medical Association conference this week have called for a workplace smoking ban.
They produced a giant "prescription" signed by doctors calling on government to impose such a ban.
Delegates at the conference also heard that
4,500 letters from doctors concerned about the negative health effects of tobacco would to be sent to the Prime Minister.
It is reported that the government is considering a ban on smoking in all public places if it wins the next election.
Previous research has linked passive smoking to increased risk of heart disease and stroke.
St George's Hospital's Professor Peter Whincup and colleagues examined the links between a blood marker of smoke exposure, called cotinine, and the risk of heart disease and stroke in more than 4,500 men.
The men were aged between 40 and 59 and came from 18 different towns across the UK. They were monitored for 20 years.
Professor Whincup's team found the men with the highest levels of cotinine in their blood, and therefore the highest exposure to passive smoke, had the highest risk of heart disease.
Higher cotinine levels were linked with a 50-60% greater risk of heart disease.
Previous studies that looked at the risk posed by living with a smoker estimated a 25-30% increased risk of heart disease.
This suggests the dangers of passive smoking may have been underestimated, according to the researchers.
The risks were particularly high when they looked over a short time scale, which suggests the link between cotinine levels and heart disease declines with time.
This means previous studies that looked at years of data could have further underestimated the risk, they said.
Professor Whincup and colleagues did not find any link between cotinine level and stroke risk, however.
Professor Whincup said: "The true effects of passive smoking may have been underestimated by concentrating on partner exposure.
"What we have here is a measure of overall passive smoking exposure.
"The effects of passive smoking are likely to be bigger and more widespread. This adds weight to the argument that we should do everything we can to minimise passive smoking exposure," he said.
Dr Tim Bowker, associate medical director of the British Heart Foundation, said: "The need for a ban on smoking in public places in the UK has never been better illustrated than by this potentially pivotal study.
"The evidence is now compelling. The government should not delay any further in introducing legislation to protect non-smokers from this unnecessary risk," he said.
Mr Ian Willmore, head of communications at Action on Smoking and Health, said: "If you regularly breathe in other people's smoke at home or at work your chances of getting heart disease may rise by more than a half.
"This is a much bigger increase in risk than was previously thought - and the difference with previous estimates seems mainly due to smoking in the workplace.
"It is time for the tobacco industry and its front organisations to stop pretending that second-hand smoke is harmless. And it is time for the Government to accept the need for a new law to end smoking in the workplace," he said.
At the British Medical Association's annual conference, Dr Peter Maguire, a consultant from Northern Ireland and the Deputy Chairman of the BMA Board of Sciences, called for a ban on smoking in public places and the workplace.
"The British government needs to have courage and follow the lead of Ireland, New York and Norway.
"I have seen that the ban on smoking in public places in Ireland has not affected business - business is booming there. Smoke free places mean life not death," he said.
A spokesman from the smoker's lobby group Forest said the jury was still out on whether passive smoking kills.
"This new report needs to be seen in perspective. It is
one of over a hundred studies on the subject and the vast majority have failed to find any connection between passive smoking and ill health. On the few occasions a link has been detected it has been too small to be significant.
"The truth is, we don't know whether passive smoking is harmful. Until passive smoking is proved to be a serious health risk it is unacceptable to use it as an excuse to outlaw smoking in public places," he said.
The Tobacco Manufacturers' Association said: "There continues to be scientific disagreement over the potentially harmful effects of passive smoking. Last year, the BMJ said the risk may have been overstated. Now a new study claims the risk has been underestimated, and only last week, Sir Richard Doll reconfirmed his view that the risk is small.
"It does appear that there is not sufficient scientific evidence to justify a total smoking ban in public places. But action should still be taken to ensure non smokers' needs are catered for by designated non-smoking and smoking areas, together with good ventilation."