A future daughter to Prince William and Princess Catherine could take the throne under a change to centuries-old laws.
A meeting in Perth of the 'realm' countries which have the Queen as their monarch, including Australia, has agreed to change the royal succession rules.
British Prime Minister David Cameron has championed the change to the 1701 Act of Settlement, which would scrap rules that put a male child ahead of his older sister in the line to the throne, and bar anyone who marries a Catholic from the royal line of succession.
'This way of thinking is at odds with the modern countries that we've all become,' he told reporters in Perth on Friday.
'I'm very pleased to say we reached a unanimous agreement on two changes.'
He said the British government would publish legislation first, before other realm countries.
Prime Minister Julia Gillard said there was in-principle agreement from the Australian federal and state governments for the arrangements.
'These things seem straightforward but just because they seem straightforward to our modern minds doesn't mean we should underestimate their historic significance,' she said.
Every state and the federal government will need to pass legislation to enact the changes.
The royal succession rule change won't affect Prince Charles or his son Prince William, both of whom are first-born children.
The realms - nations whose head of state is Queen Elizabeth - include Australia, the UK, New Zealand, Canada, Jamaica, Antigua and Barbuda, Belize, Papua New Guinea, St Christopher and Nevis, St Vincent and the Grenadines, Tuvalu, Barbados, Grenada, the Solomon Islands, St Lucia and The Bahamas.
The British law barring Catholics was originally designed to prevent the return of the Catholic Stuarts to the throne.