Personal genetic testing and DNA databases have effectively brought an end to anonymous sperm and egg donation, an expert has claimed.
The growing popularity of direct-to-consumer genetic tests is making it increasingly easy for donor-conceived individuals to discover the identity of their biological parents.
This can have "traumatic" consequences when adults who do not know they owe their existence to donated "gametes" - sperm or eggs - are suddenly confronted with the truth.
The only remedy is to make the business of donor conception as open as possible, according to a paper published in the journal Human Reproduction.
One of its authors, genealogist Debbie Kennett, from University College London's Department of Genetics, Evolution and Environment, said: "Fertility clinics need to develop robust guidelines and procedures that enable them to integrate subsequent genomic data into their existing consent agreements.
"All parties concerned must be aware that, in 2016, donor anonymity does not exist."
More than three million people worldwide are already said to have signed up for private genetic testing, often via online companies and without taking advice from health professionals.
The tests are used to obtain information about ancestry or health. In many cases, DNA profiles are stored on international genetic genealogy databases that can turn up matches with long lost relatives.
Co-author Professor Joyce Harper, from University College London's Institute of Women's Health, said: "DNA tests are increasingly being used to solve unknown parentage cases for adoptees and donor-conceived persons.
"People are finding half-siblings and even biological parents in online databases that are open to the public. A sperm donor does not have to be in the database to be identified as identification can be made from matches with other close relatives such as second or third cousins.
"Using these genetic databases, donor-conceived adults who have not been informed of their status may find out that they are donor-conceived, which may lead to traumatic breakdown of trust with parents."
Recently, there has been a move within the scientific community and more widely towards greater openness about genetic data.
Many more patients are going to know information about their genomes, or genetic codes, in the future, said the experts.
The situation is complicated by the fact that different countries, even within the European Union, have different laws on gamete donation, donor anonymity and parental disclosure.
Dr Dan Reisel, from University College London's Centre for Ethics in Women's Health, said: "These concerns make urgent a wide-ranging societal conversation about how to best safeguard and promote the interests of donor-conceived offspring and protect the rights of donors."