Prince William, while getting into his auto, said, "We're very happy but thrice anxious now".
In the message sent out by Clarence House, he said: 'We are both so pleased at the news.
Prince William has been spotted popping out to run errands like any new dad the day after the birth of their third child - unshaven and exhausted. Not only is she a royal who gives birth to possible future monarchs, but she also seems to make extremely rapid exits from the hospital after giving birth.
Yesterday, the Duke and Duchess were kept busy settling in to life as a family of five at Kensington Palace, Prince George and Princess Charlotte already having been introduced to their brother at St Mary's Hospital. Middleton's stylist Natasha Archer was also seen leaving the hospital just an hour before the new baby was born.
Take a look at some of the well wishes for the new royal baby below.
Even if he just can't wait to be King, the new prince is fifth in line to the throne, so it's highly unlikely he'll ever ascend.
The rule was changed after all 16 countries in the Commonwealth headed by Queen Elizabeth II agreed that birth order, not gender, would determine succession.
While the hospital cameo by Kate, her husband Prince William and their little prince took only a few minutes, it was a precisely choreographed performance months in the making. Prince William asked his wife on the steps, according to a professional lip reader who spoke with theMirror.
It was also announced that London's Westminster Abbey would ring its bell a full peal of Cambridge Surprise Royal. Prince William and Kate Middleton follow in the footsteps of Prince Charles and Princess Diana by presenting their babies to the public.