Buckingham Palace is understood to be supportive. One No 10 source said: “Downing Street has been working on this for five years. Buckingham Palace will not have been taken by surprise. This will welcome the crown into the modern age.”

Commonwealth leaders will pledge to amend legislation dating back to the 17th century to allow daughters of the monarch to take precedence over younger sons in the line of succession.

David Cameron will hail the agreement of the 16 Queen’s realms, the Commonwealth countries where the Queen serves as head of state, to amend “outdated” rules that also prevent a potential monarch from marrying a Catholic.

The prime minister will introduce legislation in Britain before the next general election to ensure that the changes will apply to any children of the Duke and Duchess of Cambridge. Officials say the changes will apply even if a child is born before the new legislation is passed.

Speaking before the opening of the Commonwealth heads of government meeting in Perth, where the agreement will be sealed, Cameron said: “These rules are outdated and need to change.”

In a meeting in Perth this morning, to be chaired by the Australian prime minister, Julia Gillard, the leaders of the 16 Queen’s realms will agree to amend rules that currently say:

• An elder daughter should be placed behind a younger son in the line of succession.
The order of succession will in future be determined by the order of birth. The immediate impact will place the Princess Royal, the Queen’s daughter, fourth in the line of succession behind the Prince of Wales and his two sons. At the moment the princess is 10th. The Duke of York, who is fourth, will drop to seventh.

• Anyone who marries a Roman Catholic is barred from succeeding to the crown.
This will end. The change will not affect the position of the monarch as the supreme governor of the Church of England, because Catholics will still be barred from the throne. The Church of England will remain as the established church.

• Descendants of King George II need the monarch’s consent to marry.
This will be reformed.

Cameron will tell the meeting: “The idea that a younger son should become monarch instead of an elder daughter, simply because he is a man, just is not acceptable any more.

“Nor does it make any sense that a potential monarch can marry someone of any faith other than Catholic.

“The thinking behind these rules is wrong. That’s why people have been talking about changing them for some time. We need to get on and do it.”

Downing Street has noted what would have happened if the rules had been different at key moments:

• Margaret Tudor would have succeeded Henry VII in 1509, denying the throne to her younger brother, who became Henry VIII. That raises the prospect that Henry VIII would not have been responsible for the greatest example of Euroscepticism – the break with Rome in 1533.

• Elizabeth Stuart, the Winter Queen of Bohemia, would have succeeded her father James I in 1625 instead of Charles I. The civil war, in which Charles was executed, might have been avoided.

• Queen Victoria’s daughter, Princess Victoria, would have succeeded in January 1901, rather than Edward VII. The new queen would have died less than seven months later, handing the throne to Kaiser Wilhelm II. Britain would have been ruled by the German emperor during the first world war.

The announcement in Perth comes after Cameron wrote last month to the other leaders calling for change. Legislation will have to be introduced in Britain and some of the other 15 realms to amend laws including the Bill of Rights 1688, the Act of Settlement 1700, the Act of Union with Scotland 1706 and the Coronation Oaths Act 1688.

Primary legislation will be necessary in Antigua, Canada and Saint Lucia. Papua New Guinea and Tuvalu will not need to enact their own legislation.
Gordon Brown was keen to introduce the reforms, but did not feel he could set aside enough parliamentary time.

Earlier this year Cameron played down the prospect of an imminent change in the rules of royal succession, partly because of concerns that constitutional tinkering could revive the campaign in Australia for it to become a republic.

But Downing Street believes that the Queen’s diamond jubilee next year and the marriage of the Duke and Duchess of Cambridge in April show it is time to “secure a breakthrough”.

Nick Clegg, the deputy prime minister, also supports the change. “If Prince William and Catherine Middleton were to have a baby daughter as their first child, I think most people would think it fair and normal that she would eventually become queen of our country,” he said this year.

Read More