New laws on royal succession are being fast tracked through Britain's parliament and if passed would allow a first born daughter to succeed the throne.
The Succession of the Crown Bill, which also permits an heir to the throne to marry a Catholic, will now go to the House of Lords for further scrutiny.
The changes will mean that if the Duke and Duchess of Cambridge's first child is a girl, she can become monarch even if she later has younger brothers.
Under current rules, Prince Charles will take over as sovereign when the Queen dies. Prince William, Duke of Cambridge, would succeed him.