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” 32.2 per cent 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.