A major sperm study has delivered a blow to the bedroom reputation of French men.
Gallic sperm count and quality have fallen sharply since the start of the 1990s, said scientists.
The research involving more than 26,600 men showed a "significant and continuous" 32.2% decrease in sperm concentration over 17 years. Numbers of sperm per millilitre of semen fell at about 2% a year between 1989 and 2005.
Researchers calculated that in French men with an average age of 35, sperm count reduced from around 73.6 million per millilitre of semen to 49.9 million. At the same time, the proportion of normally formed sperm declined by about a third.
The authors, writing in the journal Human Reproduction, 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. They added: "This constitutes a serious public health warning. The link with the environment particularly needs to be determined."
The scientists analysed data from semen samples collected from 126 fertility clinics throughout France. All the couples involved were seeking treatment because of female problems rather than obvious difficulties linked to sperm.
Dr Joelle Le Moal, one of the researchers from the Institut de Veille Sanitaire in Saint Maurice, 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.
"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."
The findings support other research showing similar drops in sperm concentration and quality in recent years. Some studies have suggested that environmental factors, such as endocrine disrupters - chemicals that upset hormone balances in the body - might be behind the trend.
British expert Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, said despite numerous studies over the last 20 years the jury was still out on what was happening to sperm quality. He added: "There is no doubt that this paper is a useful contribution to the literature, but I would urge much caution in its interpretation as there remain too many unknowns."