Scientists in Kiel, Germany, discovered that plastic-lined nappies significantly heat up the scrotum and they believe this could harm normal testicular development. Statistics from the Department of Health show that the average sperm count of a European male has dropped by 25 per cent during the past 25 years. One couple in six in Britain requires fertility treatment, and in a third of those couples the problem is with the male.

According to the study published in the British Medical Journal Archives of Disease in Childhood, disposable nappies raise the scrotal temperature by as much as one degree centigrade above body temperature. The research team claims this proves that normal testicular cooling mechanisms are impaired by disposable nappies. In some cases they found the cooling mechanism failed altogether. High temperatures are proven to reduce sperm count in adults.

Tim Hedgley, the chairman of the National Fertility Association, said: "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 one leaks out wetness." According to the NFA, about 27,000 British couples now seek treatment for infertility problems every year, an increase of 55 per cent since 1995.

The research team, led by Professor Wolfgang Sippell, department of pediatrics at the University of Kiel, compared the scrotal and rectal temperatures of 48 healthy boys in disposable and re-usable cotton nappies. Scrotal temperatures were considerably higher than rectal ones in disposable nappies. Re-usable nappies were consistently cooler and the rectal and scrotal temperatures were the same.

The report says: "A prolonged increase in scrotal temperature in early childhood may have an important role in subsequent testicular health and function, with implications for male infertility."