The number of men - especially students - volunteering as potential sperm donors has fallen sharply since 2000, a study has found.
Loss of anonymity may have put donors off
Researchers at the Newcastle Fertility Centre say the fall is almost certainly linked to scrapping the right of donors to remain anonymous.
They say urgent action is needed to attract more volunteers, particularly among older men who may have partners.
Details are published online by the journal Human Reproduction.
The Newcastle team examined details on over 1,100 men who applied to be potential donors between 1994 and 2003.
Nearly nine out of 10 were aged under 36, more than half were students without a partner, 85% were unmarried and over three-quarters had no children.
Nearly a third had opted out during the process, and almost two-thirds were rejected, most because their sperm was not of sufficient quality.
At the end of the recruitment and testing process, just 3.63% were accepted as suitable donors.
Crucially, just 25 potential donors came forward in 2003, compared with 175 in 1993. The sharpest fall occurred from 2000 onwards.
From 1999, the numbers who changed their mind during the application and testing process also rose.
The acceptability rate of donors also declined over the study period because of the introduction of stringent criteria aimed at improving standards of recruitment.
Lead author Dr Sudipta Paul said numbers of acceptable donors was likely to be further decreased by new rules on the handling and use of human tissues.
"On the other hand, the demand for donor sperm is likely to increase because of a possible decline in the semen quality in the general UK population."
His colleague Dr Jane Stewart said the removal of anonymity from donors, which came into force in April, had led not only to a sharp fall in volunteers, but also to a change in the donor profile.
There was a significant increase in the number of volunteers who had partners, and substantial fall in the number of students.
She said: "It is clear now that we can recruit only donors willing to be identified that we need to change our strategies to target older men in established relationships.
"Since it appears they are likely to offer help for altruistic purposes, we must continue to work to increase public knowledge of the need for donors up to the age of 40."
Dr Stewart said numbers of recruits would probably be boosted if the process for releasing personal details was clearly spelled out.
However, plans to increase payment to donors did not go far enough to make a telling difference.
The Newcastle unit is reviewing its high cut-off rate for sperm quality - but warns dropping thresholds too low will cut IVF success rates.
Dr Allan Pacey, a fertility expert at the University of Sheffield and secretary of the British Fertility Society, said many other clinics had similar problems.
"Indeed, a lot of clinics have stopped accepting enquiries from potential donors because too few men are coming forward to make it worth their while to assess their suitability.
"Many clinics are still relying on stocks of sperm that were donated anonymously and that they are allowed to use in treatments until March 2006."
Public Health Minister Caroline Flint said: "It is right that the interests of donor-conceived children should be first and foremost, and that donor-conceived people should have a right to information about their genetic origins, including the identity of their donor."
She said the government had launched a campaign to encourage more people to become egg and sperm donors.