Fertility experts have warned that new rules removing the right of anonymity from sperm donors could drive infertile couples to "backstreet services".
At least 7,000 men a year need to come forward, experts say
From April 1, children born from donated sperm, eggs and embryos will have the right to trace their biological parents.
But Alison Murdoch, chairman of the British Fertility Society, warned unlicensed services could proliferate.
She also warned couples may decide to travel abroad for anonymous donation.
Fears have already been raised that new sperm and egg donors will be deterred from coming forward because of the new rules.
But Professor Murdoch, who is also a senior lecturer at Newcastle Centre for Life, said the rules may prompt couples who want to use sperm or eggs from anonymous donors to go to unlicensed clinics in the UK or abroad to countries which may have lower regulatory standards.
"My worst case scenario is that services will go underground. Everyone will lose out that way. It's not what we want to see."
She said that, as agencies offering to collect and supply fresh, not frozen, sperm do not have to be licensed, they are not covered by the new regulations.
But while this would mean they would not have to abide by the rule lifting anonymity from donors, nor would they be inspected or obliged to screen donors for genetic problems or sexually transmitted infections such as HIV.
The first web-based agency dedicated to lesbians and single women, ManNotIncluded.com, was launched in 2002.
Although it is not licensed by the Human Fertilisation and Embryology Authority (HFEA), it does vet donors and screen donated sperm for health risks.
However, the experts warned there was nothing to guarantee that other operators would maintain the same standards.
Dr Murdoch said couples may become "fertility tourists", travelling to other countries for treatment they are unable to obtain in the UK.
While centres are available in countries such as Belgium, Professor Murdoch said she was chiefly concerned that people might choose to travel to low-standard clinics outside the European Union.
"I think we have to be very careful about people going to centres that don't have the same standards and regulatory procedures."
A spokeswoman for the Department of Health said: "Removing anonymity is the next step towards increasing the availability of information for donor-conceived children and bring donor-conception more in line with adoption.
"Earlier this year we launched a brand new campaign to encourage people to become sperm and egg donors. Donating is and should be highly valued, yet there is a lot of misinformation about what donating actually involves.
"The new campaign will help to change this by changing public perceptions and encouraging more donors to come forward."
About 7,000 sperm donor cycles are carried out each year. The figure is much lower than the 26,000 conducted 10 years ago, mainly due to new IVF techniques that have allowed more wives to be made pregnant with their husband's sperm.
In 2000-01 a total of 3,500 women were treated with donated sperm.