He stood about 2.4m tall, earned his living from street shows and died at 22.

Now researchers have found that Charles Byrne, the most famous giant in medical history, sprang from a mutant gene that first appeared 1500 years ago and is still causing excessive growth in families in the same area of Northern Ireland where he was born.

The current "race of giants" is concentrated in a small area, but researchers requested its location not be disclosed to protect residents from curious onlookers.

After the discovery, doctors screened four families from the area found to carry the same genetic mutation and identified three patients in the early stages of excessive growth who had not realised they were affected.

The four families had given rise to half-a-dozen giants as well as others affected by growth abnormalities in recent generations.

Gigantism is caused by a benign tumour of the pituitary gland which produces excessive growth hormone. If it occurs early in life it can cause a child to grow very tall, but in adulthood it causes abnormal growth of the face, hands, feet and other parts of the body in the condition known as acromegaly.

Charles Byrne, born in 1761, grew rapidly in his teens and travelled to London at the age of 19 to seek his fortune.

An etching from the period shows him with two men nearly as tall, who came from a village 12km from his. After a successful if brief career displaying his physique he seems to have drunk himself into an early grave. His skeleton is on display in the museum at the Royal College of Surgeons in London.

Marta Korbonits, professor of endocrinology at Barts and the London School of Medicine, who led the study, published in the New England Journal of Medicine, said the idea for the investigation came to her after two cousins from Northern Ireland attended her clinic in London, where she cares for 150 families affected by growth disorders.

She said: "I thought of the Irish giant and initiated the studies. Then the other families fell into my lap.

"By the time I got to the fourth I was able to tell the patient his family originated from Northern Ireland and even which village they came from.

"He practically fell off his chair."