Hitherto, rules provided that a girl born to Prince William and Kate would be known as Lady, not Her Royal Highness, and only a first-born boy could get the Royal title, The Sun reported.
The Queen - by issuing new Letters Patent - changed the rules so that her great-grandchild, due this summer, has a title suitable for a future monarch.
The alteration was anticipated, Royal expert Charles Kidd said, adding that the amendment "seemed logical" to bring to an end the discrimination against women succeeding to the throne.
