Prince William has welcomed his younger brother to the "sleep deprivation society that is parenting".
William celebrated the latest addition to the royal family with the quip, and also said he was "absolutely thrilled" about the birth of Harry and Meghan's first child.
"Obviously thrilled, absolutely thrilled, and obviously looking forward to seeing them in the next few days when things have quietened down," he said ahead of an engagement in London on Tuesday.
"I'm very pleased and glad to welcome my own brother into the sleep deprivation society that is parenting."
The boy, who has yet to be named, arrived early on Monday and is thought to have been born at Harry and Meghan's Frogmore Cottage home near Windsor Castle.
But there are some reports claiming the baby was delivered in a London hospital.
Asked if he had any advice for his younger brother, father-of-three William laughed.
"Plenty of advice, plenty of advice, but no, I wish him all the best and I hope the next few days they can settle down and enjoy having a newborn in their family and the joys that come with that," he said.
The Duchess of Cambridge said it was a special time.
"Obviously with Louis and Charlotte just having had their birthdays it's such a great time of year to have a baby, spring is in the air," Kate said.
Meanwhile the Queen was quizzed about the latest addition to the family by former Canadian prime minister Jean Chretien at a lunch for members of the Order of Merit at Windsor Castle.
"Life is good for Your Majesty?" Chretien asked.
The smiling Queen replied: "Yes, thank you."
"Congratulations. Another great-grandchild!" he added.
The Queen replied happily: "Yes, I know."
She was then asked "how many of them have you got now?" before replying "eight".
Chretien quipped: "You beat me by one."
Prince Charles has also spoken publicly for the first time about the birth, saying he was "delighted" at the arrival of his latest grandchild.
"We couldn't be more delighted at the news and we're looking forward to meeting the baby when we return," Charles said during an official visit to Germany with Camilla.
