Prince Charles says it's "a great joy" to have another grandchild, as the Duke and Duchess of Cambridge settle into life with their new son. A baby boy born on April 23 joins Prince George, four, and Princess Charlotte, two.
William declared the couple "very delighted" with the new addition to the family.
William, 35, and 36-year-old Kate are back at their Kensington Palace home in London.
Still, that hasn't stopped many from at least guessing on what the princeling's name could be.
Prince William and Kate Middleton follow in the footsteps of Prince Charles and Princess Diana by presenting their babies to the public. One of the first people he would have told is his grandmother the Queen, who is now in Windsor with Prince Philip.
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Prime Minister Theresa May led the messages of support saying: "My warmest congratulations to the Duke and Duchess of Cambridge on the birth of their baby boy".
Monday is St. George's Day, England's national day, but the baby is unlikely to be given the name since his older brother already has it. On March 21, she spoke at a symposium at the Royal Society of Medicine in London. We're guessing she's at the top of Prince William's speed dial.
He's fifth in line to the throne and, like his uncle Prince Harry (now sixth in line), he'll amiably-and perhaps gratefully-further descend the ladder when his big brother Prince George has kids. The traditional 21-round gun salute has 20 more rounds if fired from a royal park or palace, and a further 21 from the Tower of London because of its location in the City of London.
William, who is destined to become king after Charles, and Kate met at St. Andrews University in Scotland.
The name of the baby prince of Cambridge, who was born on Monday at 11.01am weighing 8lb 7oz, has yet to be unveiled.
"It's really important to show support because they need to know how much they are loved by the people", she told AFP.