Men's sperm counts have fallen by almost a third since 1989, one of the largest studies of its kind has found.
Men with low sperm counts can have problems conceiving
The findings add to the evidence which suggests a growing number of men may have problems fathering children.
Researchers in Aberdeen said their findings, based on 16,000 semen samples taken from 7,500 men, "cause concern".
They said that the average "normal" sperm count had fallen from 87 million sperm per millilitre in 1989 to 62 million in 2002 - a 29% drop.
The findings of the study, involving men attending the Aberdeen Fertility Centre, were being presented at a British Fertility Society meeting in Liverpool on Monday.
Scientists said that while the fall may be a result of more men coming forward for treatment, further investigation was needed.
They are now carrying out studies to find out if there has been a similar decrease in sperm quality, which can significantly reduce a man's chances of having children naturally.
"We cannot say that there has been a fall in male fertility on the basis of these preliminary findings," said Dr Siladitya Bhattacharya, who led the research.
"There has been an increase in men seeking treatment for male infertility, but whether this is due to a significant increase in this condition or because men are more aware of new techniques...we cannot say."
Dr Allan Pacey, an andrologist at the society, told BBC Radio 4's Today programme the study added to the "jigsaw" of evidence on a possible decline in sperm counts.
He said: "British men actually fare quite badly on the European stakes and the region of Europe which is known to have the highest sperm count is Finland."
He said genetic factors were the key to sperm counts, but environmental and occupational influences also played a part.
Stem cell research
Meanwhile, a survey of couples undergoing IVF treatment suggests many may be willing to donate any leftover embryos for stem cell research.
Research on stem cells - the body's master cells - is controversial.
Scientists believe these cells hold the key to curing a wide range of diseases from Alzheimer's to cancer.
Embryos are widely considered to be the best source of stem cells, but many groups are opposed to their use.
The survey of couples attending the Newcastle Fertility Centre found that 57% of those asked agreed to donate their surplus embryos for stem cell research.
Professor Alison Murdoch, chair of the British Fertility Society, who led the study, said providing couples with information on stem cell research was vital.
"When people understand this issue they tend to look on it favourably. Scientists should not be afraid of engaging the public on this issue."
Clare Brown, chief executive of Infertility Network UK, welcomed the survey's findings.
"The results of this study highlight the fact that couples are keen to assist others whilst going through what is an extremely difficult, both physically and emotionally, treatment personally."