Explaining his plans for a married couples allowance, shadow chancellor George Osborne told the Times the policy was about "supporting commitment".

"It's not just for married people, it's also for gay people who are in a civil partnership," he said.

"I don't preach about people's lives, and many marriages fail, but I think we know now from years of evidence that a society where more people are married is a stronger society."

But Prime Minister Gordon Brown said families with children would lose out under the Conservatives.

"I am a believer in marriage, and the institution of marriage is absolutely central and fundamental to our society," he told voters in Kirkcaldy.