Because the cells are most likely released by rupturing of the arteries in the weeks leading up to a heart attack, researchers said, the warning signs could be spotted as they build up.

The American researchers from Scripps Health said that finding a way of predicting heart attacks before they occur was considered the "holy grail" of cardiovascular medicine, and could lead to patients being screened in emergency rooms.

But British experts contended their claims, saying that European studies had already identified a much more reliable sign of impending heart attacks and questioning the strength of the new evidence.

The new study, published in the Science Translational Medicine journal, focused on a type of cell known as circulating endothelial cells (CECs), which line the arteries throughout the body Tests on 50 patients who had suffered heart attacks showed that they had on average between 12 and 51 CECs per millilitre of blood, compared with between two and five CECs in healthy people, and they were also more likely to be abnormally large and misshapen.

There are about 124,000 heart attacks in Britain each year, with more than one in three sufferers dying before they reach hospital.

Dr Raghava Gollapudi, one of the lead researchers, said: "This would be an ideal test to perform in an emergency room to determine if a patient is on the cusp of a heart attack or about to experience one in the next couple of weeks.

"Right now we can only test to detect if a patient is currently experiencing or has recently experienced a heart attack. With some additional validation, the hope is to have this test developed for commercial use in next year or two."

But Prof John Martin, a cardiac specialist from University College London, said there was much better evidence that large clotting agents known as platelets building up in the blood could cause the onset of heart attacks.

He said: "I would be very suspicious about any finding that had not been demonstrated to be predicted by taking the blood before the heart attack.

"The evidence for large platelets in the circulation being causally related is far stronger than this and is based on more credible rationale."