Fertility law should recognise the rights of children to know their biological parents and not hide their identity, an independent peer has claimed.
Independent peer Lord Alton told the House of Lords the right for children to know their biological origins should be treated as a human right.
"Sooner or later the truth will out" if parents attempt to conceal children's biological identities, he warned peers.
To highlight the potential dangers of this, the former Lib Dem peer told the House of Lords one pair of twins had gone on to marry after being separated at birth and then adopted.
Lord Alton had been told of the case by a High Court judge after the marriage was annulled.
"They were never told that they were twins," he told the Lords during a debate in December.
"They met later in life and felt an inevitable attraction, and the judge had to deal with the consequences of the marriage that they entered into and all the issues of their separation."
The revelation came during a Lords debate on the human fertility and embryology bill, which updates the legislation on fertility treatment and changes the regulation and licensing on the use of embryos in research.
The bill will allow same sex couples to be recognised as the legal parents of children conceived through donated gametes, which critics claim could prevent children learning of their biological background.
Lord Alton said the married twins' story illustrated the need for children to be informed about their biological origins.
The peer told the BBC: "If you start trying to conceal someone's identity, sooner or later the truth will out.
"And if you don't know you are biologically related to someone, you may become attracted to them and tragedies like this may occur."
The bill has also come into criticism for removing the obligation for fertility doctors to consider the "need for a father" when approving treatment.
Instead doctors will have to consider the "welfare of the child" when deciding whether lesbians or single mothers can obtain IVF.
Other criticisms have been levied at the bill's approval for the creation of human-animal hybrid embryos for use in research.
Health minister Dawn Primarolo said: "This bill will allow legitimate medical and scientific use of human reproductive technologies for research to flourish in this country, while giving the public confidence that they are being used and developed sensibly with appropriate controls in place."