Rule changes mean Catholics can become royals, and first born girls can be Queen
Female members of the Royal Family are to be given equality with men in the rules of succession to the throne, under historic constitutional changes agreed unanimously today by the 16 nations of which Queen Elizabeth II is monarch.
The changes mean that if the Duke and Duchess of Cambridge's first child is a girl, she will take precedence over any younger brothers in the order of succession.
The 16 "realms", including the UK, Canada and Australia, also agreed to scrap outdated laws which ban the spouse of a Roman Catholic from taking the throne.
The changes were announced by Prime Minister David Cameron at the Commonwealth Heads of Government Meeting, attended by the Queen, in Perth, Australia.
Mr Cameron said the historic rules were "at odds with the modern countries that we have become".
Announcing the proposed changes, he said: "Put simply, if the Duke and Duchess of Cambridge were to have a little girl, that girl would one day be our queen."
Mr Cameron said: "The great strength of our constitutional approach is its ability to evolve.
"Attitudes have changed fundamentally over the centuries and some of the out-dated rules - like some of the rules of succession - just don't make sense to us any more.
"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."
Reform of the succession rules, which date back to the 17th century, has long been seen as overdue in an age of greater equality between genders and faiths.
But the pressure for change became more urgent following the marriage of Prince William and Catherine Middleton in April, which raised the prospect of new additions to the line of succession in the near future.
Putting future princesses on an equal footing with their brothers will require amendments to a raft of historic legislation, including the 1701 Act of Settlement and 1689 Bill of Rights, as well as laws in a number of the Queen's other realms - Australia, New Zealand, Canada, Jamaica, Antigua and Barbuda, Belize, Papua New Guinea, St Christopher and Nevis, St Vincent and the Grenadines, Tuvalu, Barbados, Grenada, Solomon Islands, St Lucia and the Bahamas.
Any legislative changes will apply to the heirs of the Prince of Wales, so the children of William and Catherine will be affected whether or not they are born before the process of amending laws is completed.
Mr Cameron made clear that although heirs to the throne will in future be allowed to marry Roman Catholics, the ban on a Catholic becoming sovereign will remain, as the monarch is also the head of the Church of England.
The Most Reverend Vincent Nichols, Archbishop of Westminster and president of the Catholic Bishops' Conference of England and Wales, said: "I welcome the decision of Her Majesty's Government to give heirs to the throne the freedom to marry a Catholic without being removed from the line of succession.
"This will eliminate a point of unjust discrimination against Catholics and will be welcomed not only by Catholics but far more widely."
"At the same time I fully recognise the importance of the position of the Established Church in protecting and fostering the role of faith in our society today."
Mr Cameron chaired the meeting of 16 heads of government which agreed the constitutional changes. The Queen was represented at the meeting by her private secretary Sir Christopher Geidt, who was asked to advise her of the unanimous agreement.
Speaking alongside summit host Julia Gillard, the prime minister of Australia, Mr Cameron said the 16 realms will now work together to implement the changes simultaneously, though for historic reasons the UK will have to publish its legislation first.
He said: "People have been talking about changing the rules for some time, but when there are 16 countries sharing the same head of state and each have their own constitutional, legal and political concerns, it is absolutely right that we should all discuss this together.
"That is why I asked Prime Minister Gillard for the opportunity to chair this meeting today with the heads of government of all 16 nations.
"I am very pleased to say that we have reached a unanimous agreement on two changes to the rules of succession.
"Firstly, we will end the male primogeniture rule, so that in future the order of succession should be determined simply by order of birth, and we have agreed to introduce this for all descendants of the Prince of Wales.
"Put simply, if the Duke and Duchess of Cambridge have a little girl, that girl will one day be our queen.
"Second, we have agreed to scrap the rule which says that no-one who marries a Roman Catholic can become monarch.
"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.
"But it is simply wrong that 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.
"We agreed today that this has to change."
Mr Cameron paid tribute to the Queen's "60 years of extraordinary public service" and announced the creation of a Diamond Jubilee Trust, chaired by former prime minister Sir John Major, to help those in need across the Commonwealth.
The Queen will play "her normal role" in the legislative process required to amend the rules of succession, said the Prime Minister.
Ms Gillard welcomed the proposed changes to the laws of succession, saying: "Prime Minister Cameron, can I congratulate you on leading this initiative. Can I offer you that congratulation both as a prime minister and as a woman. And can I say I am absolutely delighted that this moment in history is happening here in Perth.
"To our modern minds, these seem like simple and very rational changes.
"That there would no longer be a discrimination against women in the way in which the line of succession works, and that we would not continue the religious prohibition against marriage to a Catholic, these things seem straightforward.
"But just because they seem straightforward to our modern minds doesn't mean that we should underestimate their historical significance, changing as they will for all time the way in which the monarchy works and changing its history.
"I'm very glad this moment in history has been made in Perth."
Downing Street said that, as well as the Bill of Rights and Act of Settlement, it was expected that amendments will have to be made to the Coronation Oath Act 1689, the Act of Union with Scotland 1706, Princess Sophia's Precedence Act 1711, the Royal Marriages Act 1772, the Union with Ireland Act 1800, the Accession Declaration Act 1910 and the Regency Act 1937, as well as possibly some others.
The change to the Royal Marriages Act will end a position where every descendant of George II is legally require to seek the consent of the monarch before marrying - something which theoretically applies to hundreds of people, many of whom may be unaware of their duties. In future, the requirement is expected to be limited to a small, defined number of close relatives of the sovereign.
A Buckingham Palace spokeswoman would not comment on the development, saying only that "it was entirely a matter for the 16 realms".