Gillard’s New Zealand counterpart John Key said the news was “fabulous” for the young royal.
“It is an important day in any young couple’s life when a baby is expected and I know that Catherine and William will be as nervous and excited as anyone,” he said in a statement.
Catherine, a “commoner” whose parents are self-made millionaires from a party supplies business, met William at St Andrews University in Scotland in 2001.
Speculation about a royal pregnancy intensified last Wednesday when the couple were given a baby’s romper suit during their first visit together to Cambridge, the historic English university city which is home to their dukedom.
William laughed and said “I’ll keep that” after accepting the tiny hand-stitched outfit with the words “Daddy’s little co-pilot”, a reference to his job as a Royal Air Force search and rescue helicopter pilot.
The baby news comes just over one year since Commonwealth nations agreed to scrap centuries-old laws barring first-born daughters from inheriting the British throne if younger male heirs were available.
Deputy Prime Minister Nick Clegg said Monday that Britain was currently in the process of overhauling the 1701 Act of Settlement.