A documentary maker claims to have up to 1,000 siblings through sperm banks.
Barry Stevens, of Toronto, says his siblings are spread across the U.S., Canada, Europe and beyond.
His revelation comes amid increasing concerns over the lax rules for sperm donors in America.
Some countries have laws officially limiting the number of children who can be born from a single donor, but in the U.S. and Canada no law exists.
Sparm banks set their own regulation, but these are often ignored.
Among the concerns over sperm banks is the practice of trying to create a ‘designer’ baby by choosing a donor from a catalog, based on eye colour, physique or IQ level.
That practice has meant some of these donors, like Mr Stevens, have now left a huge legacy.
Fertility experts are now warning that the over-reliance on one donor increases the risk of the transmission of genetic diseases and malformations.
There are also concerns about the dangers of inadvertent incest between half-siblings.
Mr Stevens, who has made films about his search for his real father and his half-siblings, said: ‘We can’t exclude the possibility that children of one donor can meet and have sex, and children, and indeed the possibility that the donor himself could have sex with his daughter.’