The Government has received final consent from all the Commonwealth realms to press ahead with legislation ending discrimination against women in the line of succession to the British throne, Deputy Prime Minister Nick Clegg said.
Mr Clegg said ministers would now introduce the Succession to the Crown Bill in the House of Commons at the "earliest opportunity" available in the parliamentary timetable.
The legislation will end the principle of male primogeniture, so that the first child of the Duke and Duchess of Cambridge will succeed to the throne, regardless of whether the baby is a girl or a boy. The legislation will also end the bar on anyone in the line of succession marrying a Roman Catholic.
"This is a historic moment for our country and our monarchy. People across the realms of the Commonwealth will be celebrating the news that the Duke and Duchess of Cambridge are expecting their first child," Mr Clegg said.
"We can also all celebrate that whether the baby is a boy or a girl, they will have an equal claim to the throne. It's a wonderful coincidence that the final confirmation from the other realms arrived on the very day that the Duke and Duchess of Cambridge made their announcement."
The new rules will apply to any child born in the line of succession after the Commonwealth Heads of Government meeting in Perth, Australia, in October 2011 when the 16 countries which have the British monarch as their head of state agreed in principle to the change.
Since then, the New Zealand government has been co-ordinating the formal consents from each realm, with assurances that they will be able to complete the necessary measures before the British legislation comes into force.
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.
On Monday night, Foreign Secretary William Hague warned that obtaining the formal legal status would not necessarily be straightforward.
"They all have their different legislative procedures and in some countries, because they have to change the constitution, other people can suggest other changes to the constitution," he said. "It's not as straightforward as it looks but clearly this has to be done quickly."