LONDON — Drinking at least one cup of tea a day could cut the risk of heart attack by 44 percent, according to new research presented Thursday.

Researchers say the beneficial results probably are due to the powerful amounts of natural substances in tea known as flavonoids, vitaminlike nutrients that make blood cells less prone to clotting.

Flavonoids also are one of the most powerful antioxidants, or substances that offset the damaging effects of oxygen in the body. Scientists have recently become excited about the potential benefits of flavonoids, which also are found in fruits and vegetables and are famously connected to the heart-healthy effect of red wine.

While earlier studies have suggested tea drinking could be good for the heart, the latest findings are the most comprehensive and indicate the most dramatic effect.

“This is, in my view, quite an astonishing outcome,” said Dr. Catherine Rice-Evans, an antioxidant researcher at King’s College, London, who was not connected with the study. “These are very exciting results.”

The study by Dr. Michael Gaziano, a heart specialist at the Harvard Medical School-affiliated Brigham and Women’s Hospital in Boston, was presented at a Royal Society of Medicine conference in London.

It examined 340 men and women who had suffered heart attacks and matched them by age, sex and neighborhood with people who had never had heart attacks. It then investigated their coffee- and tea-drinking habits over the course of a year.

The study involved regular tea from black tea leaves, as opposed to green or herbal teas. Scientists say black tea contains more powerful flavonoids than green tea, while herbal teas are not known to contain any flavonoids.

Other studies have shown that adding milk, sugar or lemon to the tea does not diminish the effect of the flavonoids. There also is no difference between drinking it hot or cold, or preparing it with loose tea leaves, tea bags or granulated crystals, according to Dr. Paul Quinlan, a biochemist who heads the Brook Bond tea company’s health research unit.

The study was adjusted for factors that could have skewed the results, such as smoking, exercise, alcohol intake and family history of heart trouble.

Total calories consumed, intake of fatty foods and body mass index — which compares the girth of people of different heights to determine obesity — was about the same across the board.

Few of the study subjects drank one beverage exclusively, so they were categorized by their strong preferences. Gaziano found that those who drank one or more cups of tea a day slashed their risk of heart attack by 44 percent, compared with those who did not drink tea. The study did not compare the benefits of one cup vs. two, three or four.

However, the question of how much tea to drink, and how strong it needs to be brewed to get the greatest heart benefits, is still open to debate.

John Folts, a University of Wisconsin heart specialist who studies the effects of flavonoids on the heart and was not connected to the study, said he doesn’t think one cup of tea a day would be enough. His studies on dogs have indicated that six cups of tea a day are needed to prevent a blood clot in the coronary artery, which causes a heart attack by blocking the flow of blood to the heart.

Scientists have not compared the flavonoid benefits of tea with those of red wine, made famous by research showing that the French, with red wine as a staple, have lower rates of heart disease despite their penchant for high-fat food.