The end of anonymous sperm and egg donation to infertile couples could encourage the growth of unregulated backstreet clinics and websites willing to circumvent new government rules, fertility doctors fear.

The registered and regulated clinics are reporting a big decline in the numbers of men willing to donate sperm in anticipation of the rule change next month. It is thought that donors, who are often students, are reluctant to place their details on record for fear a genetic child will come knocking at their door.

Yesterday members of the British Fertility Society, who run the clinics, said that the shortage of donors, together with the desire of some couples to hide the fact that their baby was born of donated sperm, could have unfortunate consequences.

"The worst case scenario is that the service goes underground," Alison Murdoch, its chairwoman, said.

Clinics which collect and freeze sperm for infertile couples must be licensed by the Human Fertilisation and Embryology Authority and will have a statutory responsibility from next month to collect information on the identity of every sperm donor, which can beseen by his genetic children when they are older.

The artificial insemination with fresh sperm is not covered by the law. Fertility doctors believe more private arrangements may be made.