LONDON (AFP) - The debate in Britain over legalising gay marriage took a surreal turn on Tuesday after a senior politician said it could result in a lesbian queen giving birth to an heir by artificial insemination.

Lord Norman Tebbit, a member of Prime Minister David Cameron's Conservative party who sits in the House of Lords, also joked that it could see him marry his own son to escape inheritance tax.

Lord Tebbit's intervention comes amid fevered debate in Parliament over a Bill to legalise same-sex marriage, which is opposed by many of Mr Cameron's Conservative lawmakers.

"I said to a minister I know: Have you thought this through? Because you're doing the law of succession, too," Lord Tebbit, 82, told The Big Issue magazine.

"When we have a queen who is a lesbian and she marries another lady and then decides she would like to have a child and someone donates sperm and she gives birth to a child, is that child heir to the throne?

"It's like one of my colleagues said: we've got to make these same-sex marriages available to all.

"It would lift my worries about inheritance tax because maybe I'd be allowed to marry my son. Why not? Why shouldn't a mother marry her daughter? Why shouldn't two elderly sisters living together marry each other?"

The comments by Lord Tebbit, who was a minister in Tory former prime minister Margaret Thatcher's government in the 1980s, sparked derision on Twitter.

But they indicate the opposition awaiting the gay marriage Bill when it goes to the House of Lords for consideration.

The legislation looks set to be approved by MPs in the House of Commons, but it must also pass through the unelected lords before becoming law.

A majority of the public back the idea of gay marriage, but critics say marriage should only be between a man and a woman.

Civil partnerships for gay couples were introduced in Britain in 2005.