The research was carried out at Duke University Medical Center

Indeed, more than half of the group were discharged from hospital after the surgery with some form of cognitive deficit.

This, as expected, improved, with only a quarter of the group having persistent problem six months later.

However, when the same group was re-checked some five years later, 42% had suffered a measurable decline in cognitive ability compared to their pre-operation state.

While it might be expected that some people lose some mental ability as they age, this group suffered a decline more than two times that found in a control group of nearly 6,000 patients of similar ages.

Cause unknown

Dr Mark Newman, who led the research, said: "Little is more devastating to patient and family than for the patient to have a successful operation that prolongs life, but diminishes the quality of that prolonged life.

"Our results confirm long-term persistence of cognitive dysfunction and the importance of preventing these deficits."

It is not known precisely what causes these problems, although suspicion falls on the heart-lung machine, which circulates oxygenated blood around the body once the heart has been stopped so the delicate operation can take place.

It is thought that tiny blood clots may form, travel to the brain and lodge in blood vessels, depriving very small areas of the brain of a blood supply.

In the future, minimally-invasive heart surgery, which does not require heart bypass, may become more common.

A spokesman for the British Heart Foundation said that while cognitive decline following heart bypass surgery was not unknown, most people recovered in the months following the operation.

She said that she hoped the study would not scare patients into refusing heart surgery, as the benefits outweighed the risks.

She said: "The introduction of beating-heart surgery should eliminate this problem."