LONDON (Reuters) - British doctors said on Wednesday they plan to inject stem cells into heart attack patients in an experimental treatment aimed at preventing heart failure and deaths.

About 100 patients will receive stem cells from their own bone marrow -- or a placebo -- within five hours of a heart attack in the study, expected to begin early next year.

"We are hoping that the patients will have an increased quality of life six months after the procedure," said Professor John Martin, of University College London who will conduct the trial.

Stem cells are master cells that can turn into any cell or tissue type. Scientists believe they could act as a type of repair system and offer new treatments for illnesses ranging from heart disease and diabetes to Alzheimer's and multiple sclerosis.

Embryonic stem cells have the most potential but their use is controversial because they are derived from early embryos. Adult stem cells have a more limited range. But cells taken from the patients themselves overcome the ethical concerns and reduce the risk of their being rejected by the body.

"All other studies have put cells into the heart in small groups of patients several days or weeks after the heart attack," Martin told Reuters.

The British study will combine the normal treatment for a heart attack, along with the stem cell therapy.

The scientists believe that if the therapy is delivered quickly after an attack it can stop the damage to the heart. Earlier stem cell studies have tried to repair the heart after it has been damaged.

The stem cells will be taken from the bone marrow of patients after treatment for the heart attack. Once the cells have been prepared, they will be injected into the body.

The trial follows earlier animal and human studies using stem cells for treating heart disease.

In a trial of 28 patients with severe heart disease, scientists in Hong Kong and Australia found the treatment improved the blood pumping ability of the heart and increased the patients' ability to exercise.

Martin said within two years doctors should know the results of the study, funded by the UK Stem Cell Foundation which supports stem cell research.

Heart disease is a leading cause of death and disability worldwide. In Britain 100,000 people die each year from heart attack. In the United States nearly 700,000 people die of heart disease each year.

Many people who survive a heart attack suffer subsequent health problems. Smoking, high blood pressure, raised cholesterol, obesity, lack of exercise and diabetes are risk factors for heart disease.