The research is all the more significant because some of the scientists responsible were sceptical of ground-breaking findings published in 1992 which showed a dramatic fall in sperm counts over the past 50 years.
The latest study demonstrates for the first time a connection between falling sperm counts and when a man was born. One theory is that the build-up of man-made chemicals in the environment - many of which can mimic female hormones - could be interfering with male embryo development.
One of the latest studies looked at 1,351 healthy sperm donors living in Paris and found an unequivocal decline in sperm counts over the past 20 years with the youngest men having the poorest semen.
A second study, by researchers in Belgium, has also confirmed declining male fertility. It found a dramatic increase over a 17-year period in the number of men producing defective sperm.
Preliminary results of the Paris and Belgium studies have excited British researchers examining the tentative link between declining male fertility and environmental pollution.
Pierre Jouannet, of the Central University Hospital of Bicetre near Paris, was known to be sceptical of an earlier study by Danish researchers indicating falling sperm counts. But now he has found that men in Paris are losing on average a million sperm a year from each millilitre of semen - a fall of some 2 or 3 per cent.
His colleague Jacques Auger said: 'If this trend continues it will give problems for fertility, especially if the sperm are poor quality.' An annual fall in sperm counts of 2 or 3 per cent was 'a lot', he said.
Richard Sharpe, male fertility specialist at the Medical Research Council's Reproductive Biology Unit in Edinburgh, said that Dr Jouannet's approach was the best test possible of the 1992 Danish study because he had set out to disprove it.
'His is the most definitive research to date,' Dr Sharpe said.
Professor Frank Comhaire of the University Hospital in Ghent, who led the Belgian survey of 360 healthy men who had given semen samples over a 17- year period to a donor clinic, said the number of young men with low concentrations of 'grade A' sperm - which swim fast and straight and are most likely to fertilise the egg - had risen from 5 per cent to 40 per cent.
'An increasing proportion of them could face fertility problems,' Dr Comhaire said.
Dr Comhaire and Dr Sharpe said the new findings should dispel lingering doubts in the medical profession that a decline in semen quality over the past few decades was a genuine phenomenon. Only last month the British Medical Journal criticised suggestions that sperm counts had fallen in the past 50 years on the basis that the Danish study was statistically flawed.
The finding that falling semen quality can be linked to the year of birth fits in with the hypotheses of Professor Niels Skakkebaek of the Rigshospital in Copenhagen, who was responsible for the Danish study, and that of Dr Richard Sharpe from Britain's Medical Research Council.
They conclude that in recent years environmental chemicals may have been playing an adverse role in the development of a male embryo's testes while in the womb.
Dr Sharpe said that the crucial period in the development of an embryo's testes was approximately between weeks six and 20 of pregnancy, coincidentally when levels of female hormones are at their lowest in pregnant women. Any extra females hormones - or chemicals that mimic their effects - during this time could disrupt normal development.
The tentative suggestion - it still has little supporting evidence - is that some man- made chemicals in the environment that are known to mimic female hormones could influence this crucial stage of development, causing the testes to produce poorer quality and fewer sperm.
Dr Sharpe said that more than 30 man-made chemicals in the environment were known to have such 'oestrogenic' properties. These include DDT, PCBs, some detergents and some common substances used in plastics. The danger is that many of them can accumulate in body fat to a concentration that is 100 or 1,000 times background levels. DDT, for instance, has been banned for a number of years but many of us have detectable levels of it in our bodies even though we have never been exposed directly, he said.
Earlier this year tap water in the London area was blamed by scientists at the Royal Free Hospital for an apparently low sperm count among some men in the Thames region, a claim strongly denied by Thames Water Authority.
Other scientists believe it is just as likely that the use of domestic products - such as floor polishers and plastic food wrappings - are exposing people to increasing levels of oestrogenic chemicals.
One problem in studying male fertility is that sperm counts can vary greatly between men, and even at different times in the same man's life. The usual range is between 20 million and 150 million per millilitre of semen. Men with sperm counts below this are classed as subfertile - or infertile if it is less than 5 million per millilitre.
A range of factors can influence sperm production, notably tight underwear, which is frequently cited to explain the decline in sperm counts. However, few researchers believe this can explain the latest findings.
Professor Skakkebaek said the growth of testicular cancer - which has trebled in the past few decades - was a measure of the damage being done to the male reproductive system in recent years.
'Everyone seems to accept that testicular cancer is increasing but is is more difficult to accept that there has been a change in semen quality. It's a more emotional subject.'Reuse content