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In a statement released by Clarence House, the Prince of Wales' official residence, the Queen and members of both Kate and William's families were said to be "delighted with the news".
Also welcoming the news was Scotland's First Minister, Alex Salmond, whose campaign for Scottish independence threatens to break apart the united Kingdom. Using the couple's Scottish titles, he said: "Congratulations & best wishes to the Earl & Countess of Strathearn. Wonderful to hear they're expecting their second baby - very happy news!"
Australian Prime Minister and ardent monarchist Tony Abbott congratulated the couple, saying: "It is an exciting time in any young family's life. People across the country were delighted to welcome the Duke and Duchess and young Prince George to our shores earlier this year.
"The young Royals' deep and growing affection for Australia was evident and people delighted in meeting the young Prince. This is an exciting time for the Cambridges and I look forward to welcoming their growing family back to Australia."
Monarchy New Zealand were overjoyed by the news although the announcement was too late for them to celebrate with high tea.
Chairman Sean Palmer posted the news on Facebook as soon as he heard. He said the royals were immensely popular in New Zealand and there was growing popularity for them among young Kiwis.
Watch: Prince William: 'Thrilled' by second baby
"We saw how excited the country was by Prince George's birth and visit here a couple of months ago so I think we'll have a similar reaction this time."
The organisation would mark the occasion in an appropriate way. When Prince George was born the country had lit up about 50 major landmarks like the SkyCity Tower in blue.
Dr Palmer said if the baby was a girl he liked the name Elizabeth but if it was a boy, he thought the royal couple might opt for something less traditional - "possibly Philip".
He hoped the Government would send a nice gift when the baby was born that highlighted our creative and cultural strengths.
"When George was born the Government sent a shawl as a gift from New Zealand and I'm sure they will send something this time as well."