Lord David Alton, a pro-life independent member of the unelected upper chamber House of Lords, said the twins were granted an annulment after a high court ruled that the marriage had never validly existed.

'It's a tragedy for the couple who are involved, a terrible tragedy. Everyone's hearts will go out to people caught up quite unwittingly in a case of incest of this kind,' Lord Alton said, according to BBC radio.

The Catholic politician, who discovered the case after talking to a judge, used it to highlight perceived deficiencies in the government's proposed Human Embryology and Tissues Bill, which is currently before Parliament.

The Bill is designed to make it easier for same-sex couples to have children through assisted reproduction, recognising the partners as legal parents of babies conceived through donated sperm and eggs.

But it contains no provision to require the identity of the donor to be disclosed, potentially meaning a child might not be told that he or she was conceived through assisted reproduction.

Lord Alton raised the case of the twins - who were born after in-vitro fertilisation (IVF) treatment - during a debate on Dec 10, details of which appeared in the British media only on Friday.

'There are implications for everybody involved, but the needs of the child will always be paramount, and it is right that we should therefore make the process as transparent as possible,' Lord Alton told the House.

'There will be more cases like this if children are not given access to the truth.'

IVF - which increases the chances of multiple births - meant such cases could become more common if the law does not require children to be told they were donor-conceived and have access to their genetic history, he said.

The identities of the twins and details of their relationship and marriage have been kept secret, but it is known they were separated soon after birth and never told they were twins. They discovered they were blood relatives only after the wedding.

Children's rights groups in Britain have argued that both biological parents should be named on birth certificates. But other groups, including those supporting gay and lesbian rights, have opposed it, saying it could cause children, including those with same-sex parents, unnecessary distress.

AFP, Reuters