Conservative peer Lord True has suggested the law needs to be changed to prepare for a married lesbian Queen who conceives using donor sperm.

Parliament is currently debating the Succession to the Crown Bill, which is intended to update royal laws to ensure that a first-born daughter to the Duke of Cambridge becomes Queen.

Under the 18th century laws governing the succession, only an “heir to the body” can succeed. The phrase was intended to mean direct biological descendents of the monarch.

Some experts now argue that advances in artificial insemination mean that the term needs to be redefined.

Lord True, who had tabled an amendment, said the rules should explicitly demand that future monarchs be the offspring of both parties of a heterosexual marriage, excluding children conceived using sperm or eggs from a donor or surrogate.

He told the Telegraph: “What happens if we have a lesbian Queen in a same-sex marriage who conceives using an egg implanted with donor sperm? The law should be clear, but this is a question that has not been thought through in the bill.”

Lord Wallace of Tankerness, a government law officer, told the Lords that after “much thought” ministers had decided against changing the bill.

“Existing laws already ensure that only a royal child born to heterosexual parents can succeed, he said.

“The laws governing the succession require that the heir must be the natural-born child of a husband and wife,” he said. “We do not believe that there is a need for this amendment.”

In response, Lord True agreed to withdraw his amendment but said he remained “troubled” by the questions raised by the government’s pending same-sex marriage bill for England and Wales – when it comes to the royal succession.