The Duchess of Cambridge's baby will ultimately accede to the throne regardless of its sex after changes in the rules of royal succession, ministers have said.
In a break with more than 300 years of English constitutional tradition, laws that would have passed the Crown to the oldest male heir of the Duke of Cambridge will not apply.
Under the previous rules of primogeniture, any male child born to the couple would have taken precedence in the order of succession over older sisters.
That would have meant that if next year's child were to be female, her place in the line of succession could have been taken by a younger brother.
At a summit in Perth, Australia, the Prime Minister agreed in principle that succession rules will change across the Queen's Realms, the 16 nations where the Queen is head of state.
In Perth, a Realms "working group" was established under the leadership of New Zealand to make sure the necessary legislation is acceptable to all countries and that the process is coordinated.
British legislation to change the rules has not yet been passed, nor have any of the other Realms done so.
Nonetheless, British ministers have insisted that the agreement is enough to ensure that the child will not be affected by primogeniture rules.
Nick Clegg, the Deputy Prime Minister, said last night that a new law to change the succession rules in Britain was in the process of being drawn up.
"We're literally working right now in government to put the finishing touches to legislation which will update the very old-fashioned rules of succession.
"We're making sure that if it is a baby girl, she can be our queen," he said.
He added: "That is a big, big change and something which the country thinks should have happened a long time ago."
The duchess's extreme morning sickness has given rise to speculation that she may be carrying twins, as hyperemesis gravidarum is associated with multiple births.
Twins have never been born into prominent positions in the British line of succession before.
Were the duchess to bear more than one child next year, the first to be born would take a higher place in the line.
Last year, Crown Princess Mary of Denmark gave birth to twins, a boy and a girl.
Before the birth, there was speculation that Princess Mary would have a caesarean section delivery, leading to reports that the presiding doctor would effectively be choosing the babies' position in the line of succession.
The babies were eventually delivered without surgery. The boy, Prince Vincent, was delivered around 25 minutes before his sister, Princess Josephine, and therefore took a higher place in the Danish line of succession.
The law on the royal succession in Britain is often referred to as the Act of Settlement, but the rules are in fact set down in several different pieces of legislation passed in the 17th and 18th centuries.
They include the Act of Settlement, the Bill of Rights, the Royal Marriages Act and Princess Sophia's Precedence Act.