Thousands of people have been wrongly told they are in danger of developing life-threatening heart diseases because of flaws in the way doctors routinely calculate the risk, according to a study of more than a million people published today.

Current estimates of the number at risk of cardiovascular diseases are 1.5 million too high, the report says, suggesting the anti-cholesterol drugs statins are massively and needlessly over-prescribed, inflating the £2bn annual bill to the NHS.

The study in the British Medical Journal made a series of other significant discoveries. It found that white middle-aged men have a lower risk of heart disease than previously thought and women from poorer backgrounds have a significantly higher risk. It also found one in three women in their 60s are at risk of heart disease, a figure previously thought to be one in four.

Concern over the misdiagnosis of heart disease will be compounded by a separate study due today from the Healthcare Commission. It will raise concern over the way the 300 healthcare trusts in England and Wales are monitoring the problem. A third of them were unable to provide details of the treatment being given to patients.

The HC said the number reported as having heart failure was 140,000 fewer than expected. While it is unsure whether the shortfall is a statistical blip, the study from the BMJ highlights how many thousands of people will probably be given the wrong treatment anyway.

Julia Hippisley-Cox, lead author of the study, said: "We are potentially missing the right people for treatment. If we use this new score it would increase treatment to deprived areas and especially to women. They are being under-treated across the board." She added that research was urgently needed into the risks for people from ethnic minorities amid fears that the current assessment scores, which were originally modelled on white populations in the US, are discriminating.

The researchers tracked 1.28 million healthy men and women aged between 35 and 74 over a period of 12 years to April 2007 and used GP records from 318 general practices.

The traditional way of calculating the risk from heart disease involves a score based on smoking, blood pressure and "good" and "bad" cholesterol, along with age and sex. The BMJ study compared this measure against a new, more sophisticated test, which also takes into account social deprivation, genetic factors and weight. It found that the former over-predicted the number of people at high risk of developing cardiovascular diseases by 35%.

It concludes that 3.2 million adults under the age of 75 are at risk of developing cardiovascular illnesses compared with the 4.7 million previously estimated.

Statins are widely prescribed at £2bn a year to try to reduce some of the 100,000 annual deaths from heart illnesses. Michael Summer, of the Patients Association, said the scoring system needed revisiting: "Anyone with any doubts should go back to their GP with this study to make sure that they really need statins."

Prof Hippisley-Cox said: "If people are being put on treatment when they don't need it, not only could there be side effects but they will be experiencing unnecessary anxiety."

The National Institute for Health and Clinical Excellence published recommendations last week that all people over 40 should be considered for statins and offered them if they are at a 20% risk of becoming ill within 10 years, though they should first be advised on healthy lifestyle programmes. Last night a spokesman said the BMJ study was published too late for the consultation. "We are currently looking at it [the research] and will determine what action to take."

June Davison, a cardiac nurse at the British Heart Foundation, said: "The most important thing is that people go to their doctors and get their cholesterol and blood pressure tested."