Kate and William's oldest child will have the right to rule, and even marry a Roman Catholic, under Britain's new succession law.

The United Kingdom's prime minister, David Cameron, set the record straight.

Put simply, if the Duke and Duchess of Cambridge were to have a little girl, that girl would one day be our queen, he said, the BBC reported. The idea that a younger son should become monarch instead of an elder daughter simply because he is a man, or that a future monarch can marry someone of any faith except a Catholic -- this way of thinking is at odds with the modern countries that we have become.

Commonwealth countries had to agree to have the rule changed. The decision was made at a summit in Australia, the BBC reported.

I'm very enthusiastic about it, Australia's Prime Minister Julia Gillard said, BBC reported. You would expect the first Australian woman Prime Minister to be very enthusiastic about a change which equals equality for women in a new area.

Cameron noted that while a monarch can now marry a Roman Catholic, he or she cannot actually be Catholic.

Let me be clear, the monarch must be in communion with the Church of England because he or she is the head of that Church, Cameron said, the BBC reported. But it is simply wrong they should be denied the chance to marry a Catholic if they wish to do so. After all, they are already quite free to marry someone of any other faith.

The change in succession rules comes at a time when just the whisper of pregnancy sets off waves for royal watchers eager to see the glamorous couple have children.

You shouldn't muck around too much with the constitution, but it's a good idea to change this at this time, royal expert Hugo Vickers told The Associated Press. It's much better to have it sorted out before any babies come along.