Commonwealth leaders have agreed to remove a centuries-old gender discrimination rule that favours first-born sons over older daughters in the order of succession to the throne.

The changes would mean that if Prince William and Kate, the Duchess of Cambridge, "were to have a little girl [as their first child], that girl would one day be our queen," British Prime Minister David Cameron said Friday.

The leaders of the Commonwealth's 16 realms — the countries that have the Queen as their head of state among the 54 total members of the Commonwealth — unanimously agreed to the succession change at their meeting in Perth, Australia.

The leaders also agreed to lift a ban on the monarch being married to a Roman Catholic.

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"Attitudes have changed fundamentally over the centuries and some of the outdated rules — like some of the rules of succession — just don't make sense to us anymore," Cameron said.

"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," he said.

Canadian Prime Minister Stephen Harper said the "obvious modernizations" were long overdue. He didn't give a timeline for when legislation will come before Parliament for approval, but said he hopes it will be adopted quickly.

The British government began reviewing the succession rules prior to William and Kate's April wedding. The old laws of succession date back more than 300 years. The current monarch, Queen Elizabeth, assumed the throne because her father, George VI, had no sons.

"With this change, it’s bringing the British monarchy in line with the practices of the continental monarchies, such as Sweden for instance," said Carolyn Harris, a teaching fellow at Queen's University in Kingston, Ont., and an expert on British royalty.

"In the past few decades, there have been a number of European royal houses that have changed their laws of succession to provide equal inheritance rights to men and women," she told CBC News.

Harris said concerns that emerged in the past were whether or not changes to introduce gender equality and marriage to Catholics would be retroactive, and if that would change the current order of succession.

"But they’re setting it up so it's descendants of Prince Charles, who has two sons. So, basically, it’s addressing members of the Royal Family who have not yet been born, the future children of Prince William and Prince Harry," she said.

With files from The Canadian Press and The Associated Press