The birth of Britain's royal baby girl is a cause for celebration not just across the Commonwealth realms but also for campaigners as the princess benefits from an historic change in the succession laws.

Prince William and his wife Kate's daughter, who was born on Saturday, is the first major royal who cannot be overtaken in the line of succession to the throne by any future younger brothers.

British Deputy Prime Minister Nick Clegg announced on March 28 that laws to end centuries of male primogeniture had taken effect, following the enactment of the required laws in Australia.

The princess is fourth-in-line to the throne, after Queen Elizabeth II's eldest son Prince Charles, Charles's eldest son Prince William, and her older brother Prince George.

She would have assumed the same position in the succession order under the old rules, but now cannot be shunted down the list should William and his wife Kate have a baby boy in the future.

Her great-grandmother Queen Elizabeth II came to the throne by virtue of only having a younger sister, princess Margaret.

The new arrival pushes William's younger brother Prince Harry, 30, down to fifth in line to the throne.

Ending male primogeniture had been talked about for decades and William and Kate's wedding in 2011 jolted governments into action to ensure that if their first-born was a girl, it could not be kept off the throne by a younger brother.

The prime ministers of Britain, Australia, Canada, New Zealand and the 12 other countries that share the same monarchy reached agreement in principle at the Commonwealth summit in Perth, Australia, in October 2011.

By sheer coincidence, formal agreement to proceed with legislation in Britain was received on the same day that Kate's first pregnancy was announced.

Sped through parliament in Britain, the Succession to the Crown Act 2013 gained royal assent on April 25 that year.

The law change had to be unanimous and identical in each country to avoid the possibility of ending up with different monarchs in different states.

Backdated to October 2011, the new law only came into effect when all 16 Commonwealth realms had ratified it.

The arrival of a prince in 2013 bought time to overcome legal wrangles, particularly in Canada and Australia.

The announcement of Kate's second pregnancy spurred efforts to complete the law change, and now any girls born after October 28, 2011 -- the date of the "Perth Agreement" -- cannot be overtaken in the royal pecking order by the arrival of a younger brother.