Twins separated at birth and adopted by separate parents later married each other without realising they were brother and sister.

The siblings were recently granted an annulment in the high court's family division.

The judge ruled that the marriage had never validly existed. Marriages can be annulled if one of the parties was under 16 at the time, if it is a bigamous union, or if the couple are closely related.

The identities of the British pair and the details of the relationship have been kept secret, but it is known that they were separated soon after birth and were never told they were twins. They did not discover they were blood relatives until after the wedding.

The case was uncovered by Lord Alton in a conversation with a high court judge.

He used it to highlight perceived deficiencies in the human embryology and tissues bill going through parliament.

Lord Alton argues the twins' marriage supports the need for children to know the identity of parents.

Legislation in the human embryology and tissues bill, he said, threatened to undermine children's rights to know these details.

"The right of children to know the identity of their biological parents is a human right. There will be more cases like this if children are not given access to the truth. The needs of the child must always be paramount."

However, the legislation to include on an individual's birth certificate whether they were donor-conceived was branded "a step too far" by Dr Allan Pacey, secretary of the British Fertility Society and senior lecturer in andrology, the study of diseases particular to males, at the University of Sheffield.

His main concern was that legislation would stigmatise individuals. "It's your business, not anybody else's, whether or not you are donor conceived," he said. "What we need to do is encourage parents to talk and just be honest."

Pacey was also worried that would-be parents would "take matters into their own hands" to try to avoid recording that a child was donor-conceived, such as using a friend to donate sperm or going abroad.

He said it was essential to see the case in context. He said about 3.5% of children are not their father's children, with the number rising to as much as 20% in socio-economically deprived areas.

The risk of donor conceived children unknowingly having children together was very small, Pacey said. "It is much smaller than it would be of marrying your half brother or sister in an inner-city housing estate," he explained.

Olivia Montuschi, founding member of the Donor Conception Network, said many people would try to avoid the legislation by not reporting births of donor-conceived children to their clinics.

Quoting a survey conducted by her organisation, she said many parents viewed the new legislation as an invasion of the privacy of donor-conceived children. Almost 95% of respondents to a survey conducted among the organisation's 1,100 members said new legislation would be an invasion of privacy for their children. "Donor conception has really only come out of the closet in very recent years," said Montuschi. "But, in the future, we see everything being very much more open, potentially with donors and recipients meeting and being in an ongoing relationship." Keith Reed, the chief executive of the Twins and Multiple Births Association, said the case reinforced the need to raise multiple birth siblings together. "In the case of adoption, we recommend multiple birth siblings are placed with the same family," he said.

The Lords will vote on the revised bill on Tuesday.