The incidence of testicular cancer has increased in the past 50 years. In Denmark the number of cases has risen three-fold.

There is also evidence to suggest that sperm quality has reduced in the same period, leading to an increase in low fertility.

Dr Henrik Moller and Dr Niels Skakkebęk, from the Danish National Research Foundation, believe that the same underlying factors may be responsible for the two trends.

One possible factor may be contamination of water supplies by sex hormones such as oestrogen from the contraceptive pill.

Recent research also suggests that a wide range of hormone-like chemicals found in plastics, packaging and other products may be linked to a decline in sperm quality.

Writing in the British Medical Journal, they speculate that these factors may take effect at an embryonic stage.

Big variations

Dr Moller said: "We do not know exactly which factors are responsible, but several pieces of evidence point indirectly to environmental causes increasing incidence over time."

These include an enormous variation between populations. For example there is a five-fold variation between Denmark and Finland.

Men should check their testicles regularly

The researchers also found that men born in World War II in Denmark, Norway and Sweden were much less likely to develop testicular cancer throughout their lives, suggesting that harmful agents were not present at the time of their gestation.

Dr Moller said: "Testis cancer is most probably a condition that is established at an early time during foetal life when the male genital organs differentiate.

"At this time the future fertility potential of the man may also be compromised, for example by inadequate formation of Sertoli cells in the gland. The two phenomena may go together.

Testicular cancer is associated with low birth weight, and possibly with retarded growth in the womb.

Gentic factors

The research was based on telephone interviews with 514 men with cancer and 720 controls.

It was found that men who had fewer children than was statistically expected were at a greater risk of developing testicular cancer.

Professor Ian Craft, director of the London Fertility Centre, said it was likely that both testicular cancer and low sperm counts were caused by a combination of environmental and genetic factors.

He said: "Whether it be oestrogens or toxins from pesticides that are responsible, it seems that polluting our atmosphere, food and drink is not doing us any good at all.

Professor Craft said the impact of environmental factors had been demonstrated by the use of gossypol, a cotton seed oil, in Chinese cooking.

The compound had been shown to reduce sperm counts and impair ovary function.