Royal succession rules in Britain and 15 other realms where the Queen is head of state were finally overhauled yesterday to give women equal rights.
New rules on royal succession that have now come into force, remove male bias and discrimination against Roman Catholics.
If the Duke and Duchess of Cambridge's new baby is a girl, she will follow Prince George and become fourth in line to the throne and will not be overtaken by any future younger brothers.
However, a Roman Catholic royal still cannot become the monarch.
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Deputy Prime Minister Nick Clegg confirmed the Succession to the Crown Act had come into force in a written ministerial statement, the BBC reported.
The Act was passed by Parliament in 2013, but all the countries in which the Queen is head of state had to pass any necessary legislation before it took effect.
The changes to the rules of succession were rushed through Parliament ahead of the birth of Prince George in 2013.
Under the former rules of male primogeniture, royal sons took precedence over their female siblings, including first-born royal daughters.
The act also replaces the Royal Marriages Act 1772 and means that only the first six in line to the throne need the Queen's consent to marry.
This replaces a long-standing procedure which affected any distant royal relations who are descendants of George II.
Six realms in addition to the UK legislated for the changes: Australia, Barbados, Canada, the Grenadines, New Zealand, St Kitts and Nevis and St Vincent.
Clegg wrote: "The act reflects this government's emphasis on equality by removing centuries of discrimination on both religious and gender grounds. The act puts in place succession laws that are fit for the 21st century and for a modern constitutional monarchy.