Four out of five adults with a long-term lung disease do not know they have the condition despite it being potentially fatal, according to research published today.

The first large-scale study on the prevalence of chronic obstructive pulmonary disease (COPD) reveals that only 18.8% of sufferers have had it diagnosed and only half (46%) of those in its severe stages know they have it.

The lack of diagnosis is a concern because smokers, who are particularly prone to it, can halt its progression if they are diagnosed sufficiently early and stop smoking. COPD is the sixth most common cause of death in England and Wales and kills more than 30,000 people each year.

Researchers at Cancer Research UK studied the results of saliva and lung function tests from 8,215 people aged over 35, and questioned them on whether they had ever been diagnosed with the condition or with asthma, for which it is frequently mistaken. A total of 1,093 people had the disease, indicating that 13% of this age group - or 3.7 million people - are affected.

The scientists, whose work is published online in Thorax, found that more than 30% of sufferers smoked and a further 35% were former smokers. But they warn that smokers often dismiss the symptoms of COPD - a "wet" cough usually accompanied by a lot of phlegm, tightness in the chest, and wheezing when running for a bus or climbing stairs - as an inevitable side effect of the habit.

In fact, COPD can kill. Lung capacity deteriorates after the tissue becomes inflamed and sufferers have insufficient lung capacity to breathe properly.