One in 500 people suffers from a heart condition that is the commonest cause of sudden death in fit young people and athletes, say doctors.

The condition, hypertrophic cardiomyopathy, causes the thickening of the heart wall and has been blamed for the deaths of several well-known athletes. People with the defect have a 1 per cent risk of dying each year.

Marc-Vivien Foe, the former Manchester City and West Ham footballer who collapsed and died in June 2003 while playing for Cameroon, was found to have had cardiomyopathy. The condition also killed Daniel Yorath, the 15-year-old son of Terry Yorath, a former international footballer and one-time manager of Wales, during a kickabout at his parents' home in Wales in 1994. Cardiomyopathy is caused by a genetic defect that produces an abnormal heart muscle protein. The thickening of the heart wall that results compresses the left ventricle leaving less space for the blood to be pumped.

It can cause breathlessness and chest pain on exertion but sportsmen and women tend to misread the signs and incorrectly conclude that they are unfit and need to work harder.

Writing in the British Medical Journal, Perry Elliott, from University College London, and Professor William McKenna, from the Heart Hospital, London, say that the identification of patients at risk of the condition is improving. It can be treated with drugs, surgery or an implantable defibrillator that shocks the heart when an abnormal rhythm develops.

*The number of people diagnosed with heart disease is rising even though deaths are falling, according to the British Heart Foundation.

Some 2.7 million people, or 12 per cent of the population, are estimated to be living with heart disease, compared with 7 per cent of the population in 1989.

Deaths from heart disease fell to 117,500 in 2002, down from 121,000 in 2001 and almost half of the level in 1988. Despite the fall, the UK has one of the highest heart disease death rates in Western Europe, exceeded only by Finland and Ireland.

Professor Sir Charles George, the foundation's medical director, said : "That number looks likely to rise if we allow complacency and inactivity to ruin our lives."