The abnormality was found to occur in approximately a quarter of infertile men studied by scientists from Australia and Singapore. The men had no other apparent reason for a low sperm count.

The gene responsible controls the function of a receptor involved in sperm production.

Normally, the receptor detects the presence of the male sex hormone testosterone.

However, the abnormal form of the gene prevents the receptor from sensing the presence of testosterone properly.

The mutation in the gene involved an abnormally high number of repeating units in its make up. The longer the repeat lengths were, the worse was the problem of low sperm production.

Infertility is a mystery

Alan Trounson, deputy director of the Monash Institute, where part of the research was carried out, said the cause of most male infertility was unknown.

He said: "If we could pick up a quarter of it by explaining it on this, it would have been a substantial inroad."

Extreme cases of the repeating units were connected with Kennedy disease, a very severe degenerative neuromuscular disorder which appears in 30 to 40-year-olds.

Researchers are now looking to see if they can stop the transmission of the gene to the children of men who have had IVF (in vitro fertilisation) treatment, especially through the process of intracytoplasmic sperm injection or ICSI.

The process involves injecting a single sperm into an egg and transferring the resulting embryo into a woman's womb.

The scientists are checking for variations in the sperm of men who have the expanded repeating sequence to see if embryos from their sperm could be sorted out on the basis of a normal repeating length.

Dr Amin Gorgy, clinical director of the London Fertility Clinic, described the research as "interesting".