The sperm count in French men has dropped by nearly a third in less than 20 years.
In addition, there was a "significant" 33.4% decrease in the percentage of normally formed sperm over the same period, scientists said.
A study published in Human Reproduction revealed the sperm count fell about 1.9% a year between 1989 and 2005 while the quality of sperm also declined.
More than 26,600 men took part in the study over the 17-year period.
"To our knowledge, this is the first study concluding a severe and general decrease in sperm concentration and morphology at the scale of a whole country over a substantial period," wrote one of the report's authors, epidemiologist Dr Joelle Le Moal.
"This constitutes a serious public health warning. The link with the environment particularly needs to be determined.
"This is the most important study carried out in France and probably in the world considering that you have a sample that's close to the general population."
For the average man of 35 the number of spermatozoa dropped from 73.6 million to 49.9 million per millilitre, the study showed.
But, Dr Le Moal pointed out, the sperm count remained within the norm for fertility used by the World Health Organisation , which is over 15 million per millilitre.
The journal points out that the study is important not only because of its size - it is probably the largest studied sample in the world - but because it supports other studies which suggest a similar fall in other countries.