THE reproductive health of the average man has fallen sharply in the last two decades, a study has shown, as the number and quality of sperm decreases drastically.
A comprehensive study into the reproductive health of 26,600 men found sperm concentration has decreased by a third since the 1990s.
The findings are so significant experts have warned action must now be taken to avoid significant fertility problems and the average family size decreasing.
The study, which was carried out in France, found there had been a ''significant and continuous'' 32.2pc decrease in sperm concentration over 17 years.
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.
At the same time, the proportion of normally formed sperm declined by about a third.
Writing in the journal Human Reproduction, the French authors 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.''
More promisingly, the study also showed the proportion of active or ''motile'' sperm rose slightly from 49.5pc to 53.6pc.
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.
Dr Le Moal said: ''Impairments in the quality of human gametes (male sperm and female eggs) can be considered as critical biomarkers of effects for environmental stresses, including endocrine disrupters. Firstly, this is because gametes are the very first cells from which human beings are built up during their lifetimes.
''According to the theories about the developmental origins of health and diseases, early exposures may have an impact on adult health.''
These effects can be passed down generations by the way they impact inherited DNA, she added.
A national monitoring system to assess French sperm quality is now being planned by the scientists.
Professor Richard Sharpe, from the University of Edinburgh, said: ''The take-home message from the study is extremely simple - sperm number and sperm quality has declined progressively over the study period.
''In the UK this issue has never been viewed as any sort of health priority, perhaps because of doubts as to whether 'falling sperm counts' was real. Now, there can be little doubt that it is real, so it is a time for action.
"Doing nothing will ensure that couple fertility and average family size will decline below even its present low level and place ever greater strains on society.''
Hannah Furness, Telegraph.co.uk