The treatment halved the extent of what would usually have been permanent scarring on the heart and led to the growth of new heart muscle.

However, it produced no significant change in ejection fraction – a measure of the heart's pumping capacity.

The Caduceus trial studied 25 patients, with an average age of 53, who had suffered a heart attack in the previous month. Seventeen received coronary artery infusions of 12 to 25million stem cells taken from healthy tissue in their own hearts. The remaining eight underwent standard care. A year later, the proportion of the heart scarred in patients who had the stem cell treatment had been reduced from 24 per cent to 12 per cent. No change was seen in patients who had the usual treatment.

Professor Eduardo Marbán, director of the Cedars–Sinai Heart Institute in Los Angeles, who led the US team, said: "This discovery challenges the conventional wisdom that, once established, scar is permanent and that, once lost, healthy heart muscle cannot be restored."

The study was published in an online edition of The Lancet medical journal.

Further studies will need to test for long–term improvement in patients.

Professor Jeremy Pearson, associate medical director at the British Heart Foundation, said: "It could be great news for heart attack patients who face the debilitating symptoms of heart failure."

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Scarred hearts can be restored to health with stem cell treatment, say researchers