This research is quite staggering

Tim Hedgely, chairman of Issue

Scientists at the University of Kiel found that the plastic lining in these nappies increased the temperature of the scrotum by one degree.

High scrotal temperatures are known to reduce sperm counts in adults.

The German scientists believe their discovery could explain the rise in male infertility and drop in sperm counts over the past 25 years.

Cotton nappies

The average sperm count of European men has dropped by 25% over the last 25 years and around 27,000 British couples seek infertility treatment every year.

The scientists tested the scrotal temperatures of 48 baby boys over two 24-hour periods. Each of the babies wore either disposable plastic lined nappies or cotton nappies.

They found the disposable nappies impaired normal testicular cooling while the reusable nappies had no effect on temperature.

The scientists added that the temperature of the scrotum in boys with disposable nappies could be expected to increase further when they suffered from fevers.

Long-term effects

Writing in the Archives of Disease in Childhood, which is published by the British Medical Journal, the scientists said their findings were significant.

"It seems to be possible that a prolonged and continuous elevation of testicular temperature by a mean of one degree can affect the maturation of the infant testis."

They added: "Plastic nappy use may have a negative long-term effect on testicular maturation, spermatogenesis and, in addition, may facilitate the development of testicular cancer."

Tim Hedgely, chairman of the fertility charity Issue, commented: "This research is quite staggering and could be of immense importance to us.

"There is no question that a disposable nappy creates a warm, damp environment for a child to sit on while something like a cotton towel leaks out wetness."

But Peter Stephenson, spokesman for the Absorbent Hygiene Products Manufacturers Association, which represents makers of disposable nappies, dismissed the study.

"There is no evidence to support the assertions made by this study."