A comprehensive research into the reproductive health of 26,600 men in France found sperm concentration has decreased by a third since the 1990s.
The study found there had been a "significant and continuous" 32.2 per cent decrease in sperm concentration over 17 years, 'The Telegraph' reported.
Numbers of sperm per millilitre of semen fell at about two per cent a year between 1989 and 2005, with researchers calculating the average 35-year-old man would see his sperm count reduced from around 73.6 million per millilitre of semen to 49.9 million.
The proportion of normally formed sperm declined by about a third at the same time.
French researchers said the study was the first to identify a long-term "severe and general decrease" in sperm concentration and quality at the scale of a whole country.
"This constitutes a serious public health warning.
The link with the environment particularly needs to be determined," researchers said.
Scientists analysed data from semen samples collected from 126 fertility clinics throughout France.
The couples involved were seeking treatment because of female problems rather than obvious difficulties linked to sperm, the report said.
"The decline in semen concentration shown in our study means that the average values we have for 2005 fall within the 'fertile' range for men according the definition of the World Health Organisation," said researcher Dr Joelle Le Moal from the Institut de Veille Sanitaire in Saint Maurice.
"However, this is just an average, and there were men in the study who fell beneath the WHO values. The 2005 values are lower than the 55 million per millilitre threshold, below which sperm concentration is expected to influence the time it takes to conceive," Le Moal said.
The study also showed the proportion of active or "motile" sperm rose slightly from 49.5 per cent to 53.6 per cent.