The first child of the Duke and Duchess of Cambridge could benefit from planned changes to the rules on succession, even if he or she is born before the law is altered.
Prime Minister David Cameron has written to his opposite numbers in the 15 other Commonwealth countries which have the Queen as their monarch to sound out their views on modernising rules which date back to the 17th and 18th centuries.
Mr Cameron wants to end the ban on spouses of Roman Catholics ascending to the throne and to scrap the rule which says that male children take precedence over their sisters in the order of succession.
Any change is expected to require the agreement of all 16 Commonwealth "realms" - the UK, Australia, New Zealand, Canada, Jamaica, Antigua and Barbuda, the Bahamas, Barbados, Grenada, Belize, St Christopher and Nevis, St Lucia, the Solomon Islands, Tuvalu, St Vincent and the Grenadines and Papua New Guinea.
And it will involve amendments to some of Britain's key constitutional documents, such as the Bill of Rights and Coronation Oath Act of 1688, the 1701 Act of Settlement and the 1706 Act of Union with Scotland, as well as changes to legislation in Her Majesty's other realms.
Mr Cameron is to discuss his proposals with fellow leaders when he attends his first Commonwealth Heads of Government summit in Perth, Australia, later this month, and has asked them to undertake the necessary preparations to be able to state their views then. Officials from the countries involved will work on the issue in the run-up to the summit.
Downing Street acknowledged that the process will take some time, but said that the changes would be drafted in such a way that any children of Prince William and his new wife Catherine are included.
It is understood that legislation will refer to the descendants of the current Prince of Wales, meaning that William's first child would follow him in the line of succession, whether a girl or a boy.
However, the current generation of royals would not be affected, so Princess Anne would not leapfrog her younger brothers Andrew and Edward to become fourth in line to the throne.
Downing Street said that Buckingham Palace was "aware" of the Prime Minister's letter, but the Queen was unlikely to be involved in any discussions at Perth. Changes to legislation were "a matter for Parliament", said Mr Cameron's official spokesman.