The breakthrough by American scientists means that gene therapy could eventually be the first line of treatment for people with clogged arteries instead of heart surgery.

The researchers from Tufts University in Boston injected a gene that controls the growth of new blood vessels directly into the hearts of 16 men.

The patients had all suffered heart attacks and had blocked arteries.

Most had chronic chest pain which prevented them leading a normal life.

Straight to the heart

The gene, known as VEGF - vascular endothelial growth factor - instructs the body to grow new blood vessels.

It was injected straight into the patients' hearts through a small cut in the chest.

The operation lasted about an hour and the patients were able to leave hospital within days.

The research team, led by Dr Jeffrey Isner, reported its findings to the American Heart Association this week.

Gene therapy has been used previously to grow blood vessels in the legs to relieve the effects of blocked arteries, but this is the first time the treatment has been used on the heart.

Invisible

Dr Isner says it is impossible to see the new blood vessels with current technology because they are so small, but tests suggest renewed blood flow to the heart.

He adds that a wider trial will not be possible until the long-term effects of the treatment is assessed.

"The big question is how long it is going to stay," said Dr Isner.

He added that heart surgery, such as angioplasty and bypass operations, do not work on about 250,000 Americans a year who suffer from blocked blood flow.

"For these patients there is currently no other treatment option," he said.

So far, none of the VEGF patients have developed any serious side effects.

Professor Brian Pentecost, medical director of the British Heart Foundation, said: "This is potentially enormously exciting, but we need to be sure we are not increasing the risks."

Virus

Another research team, based at Cornell University in New York, has carried out similar experiments, but has got the VEGF gene into the heart by transporting it in a cold virus.

Viruses are often used to carry genes into cells because they function by injecting genetic material into healthy cells.

The virus was disabled so that it could not cause any harm to the 14 trial patients.

Another team at Harvard University have been using gene therapy to see if they can make the effects of bypass surgery last longer.

They found that people who underwent normal bypass surgery were twice as likely to have their new arteries clog up than those who had genetically engineered bypass grafts.