At a meeting of the British Commonwealth in Perth, Australia, all 16 countries that claim Queen Elizabeth II as head of state unanimously voted today to change the rules of succession.

British Prime Minister David Cameron said the constitutional changes to remove gender discrimination had been agreed upon by the Commonwealth nations.

The current rules discriminate against females, and prevent future monarchs marrying Catholics.

Individual parliaments must now pass appropriate legislation. Cameron plans to pass the law in the United Kingdom before the next general election.

The change means that the Duke and Duchess of Cambridge Prince William and Kate Middleton’s first child will be the monarch, regardless of gender.

Carolyn Harris, a teaching fellow and royal historian at Queen’s University in Ontario, Canada, said today’s move was a culmination of years of discussion of succession change. “It brings the British monarchy into line with other European monarchies,” she said.

Harris told ABC News this morning that the Swedish Parliament had made the change to oldest child in 1980 in the interest of gender equality despite the king’s protest. The Netherlands followed suit in 1983, Norway in 1990, Belgium in 1991 and in 2009 came Denmark.

She said Spain has been discussing a change to its succession laws but had decided it wasn’t an urgent matter because the king and queen have two daughters. In Japan, where the crown prince and princess have a daughter, Harris said, parliament had discussed succession laws but it was met with great concerns about changing tradition

In Saudi Arabia, however, where the role of king is passed from brother to brother, Harris said there was no possibility of women succeeding the throne. And although Thailand’s princess was more popular than her brother, the crown prince, Harris said that had not led to succession reform.

The new succession rules for the British Commonwealth nations reflect modern ideas about gender equality, Harris said, and the recognition that women who became queen in the British monarch had held long and successful reigns.

“A lot of those queens that England has had have become iconic figures,” she said. “The current queen is very popular as well. There’s quite a positive association with female rulers in the United Kingdom.”

ABC News’ Enjoli Francis contributed to this story.