Taking an aspirin a day to ward off heart attacks is ineffective for the "worried well" and can cause serious harm, a study has shown.

The finding challenges calls for people over 50 to be prescribed aspirin as a matter of course because of its blood-thinning properties. It was found that instead of bringing about a significant reduction in heart attacks, a daily dose of aspirin increased the risk bleeding in the stomach.

Professor Gerry Fowkes, presented the University of Edinburgh research at the European Society of Cardiology congress in Barcelona, where more than 30,000 heart specialists from around the world gathered.

He said: "Our research suggests that aspirin should not be prescribed to the general population, although it does have benefits for people with established heart disease or other conditions." At least six previous studies have indicated that frequent doses of aspirin could lower the risk of heart attacks and have prompted many of the "worried well" to take the pills.

The Edinburgh trial looked at 3,350 patients aged 50 to 75 who were thought to be at risk of heart disease but had not showed any symptoms. Over eight years, 181 of those taking aspirin on a daily basis had heart attacks or strokes compared to 176 who were taking placbos.

Moreover, 2 per cent on aspirin were taken to hospital with gastro-intenstinal haemorrhages compared to 1.2 per cent of those taking placebos.

Professor Peter Weissberg, medical director of the British Heart Foundation, said aspirin has known benefits but there are dangers.

He said: "Because it's been around a long time people think "it must be safe and it can't do me any harm".

"They are taking it "just in case" but it's much more dangerous than some other drugs which people get concerned about, like statins.'