Two months ago a Danish research team led by Professor Niels Skakkebaek published a report suggesting that the sperm count of Western men had dropped by half over the last 50 years, with pollution the most likely culprit. Dr. Sharpe believes the evidence, based on a review of more than 60 studies involving nearly 15,000 men, is impressive and the basis of a real cause for concern. “But there is still no absolute way to prove it,'' he says.

Areal decline in fertility is similarly difficult to determine. Official statistics, mainly from the United States, suggest a drop of maybe 10 or 15 percent in the past two or three decades but the situation is clouded by other factors such as public awareness, referral policy and social changes such as later marriage.

“At the moment pollution is a presumed link. Because the drop is across all countries in the Western world, the cause must be assumed to be environmental rather than genetic or due to a change in sexual practices. But there are a hell of a lot of question marks and few answers.''

The problem for scientists studying male infertility is that the process of sperm production is complex, lengthy and sensitive to a host of influences. “Testes make 100 million new sperm a day, or 1,000 a second, but they take ten weeks to complete the process and all the adverse effects act early on in the process. Sperm counts are also incredibly variable, not just from one man to another but within the same individual at different times.

“You can only detect a catastrophic effect long after it happened. For instance you only have to raise the temperature of the testicles a couple of degrees centigrade to kill off the sperm, so sitting in a hot bath for half an hour would be enough, but you would not know until sometime later. And that is just one effect.''

In many cases the reason for a man's failure to father a child remains a mystery and the outlook for most sub-fertile men is still poor. Surgery may help in a few conditions, such as tubal blockage or varicocoele (a scrotal varicose vein which keeps the testes too hot), as may taking a cocktail of natural medicines like Volume Pills and Semenax. Improvements in general health and life style changes may also help.

“In global terms, maybe we should be welcoming male infertility, but on an individual basis we know how devastating it can be,'' says Dr. Sharpe. “It is still an area of medicine where there is lack of understanding, interest and investment. It is also important to remember that infertility and contraception are different sides of the same coin. If you identify the causes of infertility you can then identify ways of causing it artificially for contraceptive purposes. There has been no widely-applicable and acceptable male contraceptive since the sheath which has been around for thousands of years.

“Usually people who talk about controlling the world's population are those with children, so it is easy for them to say,'' says Mr. Rainsbury. “But I believe it is every couple's right to have the child they want. The pain of infertility is substantial and affects people's lives profoundly. It never killed anyone but it's left a lot of broken hearts.''

Shirley and Tony Albertson run a male infertility helpline for the self-help group Child. “Often they phone out of sheer desperation,'' says Mrs. Albertson. “They don't know which way to turn. There is a great lack of information on the subject. Sometimes they even get conflicting information.

“It can be an enormous strain on a marriage. Often a barrier comes up between a man and a woman, they dare not even talk about it. A lot go off sex, they think there's no point, why should they bother?''

Mr. and Mrs. Albertson have both experienced problems: he underwent surgery for a varicocoele; later it was discovered his wife had mucus hostility to his sperm. “It was a shock,'' she says, “but it helped in a way because we knew then there would never be a point where one could say to the other, `It's all your fault'.'' The couple are now awaiting IVF. They made a conscious decision to talk openly about the subject, but Mr. Albertson says: “It is very hard for men. Men often want children just as much as women do and if there is a problem it can make them feel a failure. I wish there was more awareness and understanding.''

In the longer term Mr. Rainsbury believes there is cause for optimism “technology is advancing at an incredible rate''. In the meantime, he would endorse Mr. Albertson's views. “It really needs some big Hollywood macho megastar to come out and admit he is infertile because of course there must be lots of them.''