Holly Willoughby, Liam Payne and Ellen DeGeneres are among the celebrities who have sent their congratulations to the Duke and Duchess of Cambridge after the arrival of their third child.
The royal couple welcomed a son, a brother for Prince George and Princess Charlotte, at 11.01am on St George's Day.
The proud parents later showed their newborn son off to the world on the steps of the Lindo Wing at St Mary's Hospital.
TV star Willoughby posted a picture of the trio on Instagram and said they were "such a gorgeous family".
"Welcome to the world little prince," she said.
Payne, who has a baby son with his partner Cheryl, tweeted: "Congratulations to her Royal Highness the Duchess of Cambridge Kate and the Duke of Cambridge, Prince William on the birth of their baby boy!"
US talk show hots DeGeneres said: "It's a boy! Kate Middleton's new baby is now 5th in line to the throne, and season 43 of "The Crown."
"Congratulations to all!"
Comedian David Walliams quipped: "Prince Harry just got demoted again."
Former world heavyweight champion boxer Frank Bruno, Hollywood actress Mia Farrow and TV presenter Paddy McGuinness were also among the celebrity well wishers on social media.
"Congratulations!!! Welcome to the world little fella," McGuinness said.
Comedian Sarah Silverman offered the royals some suggestions for a name, tweeting: "Royal baby name pitches: Joseph, Michael, Henry, Idris, Sandy, Clyde, Butch, Grandpa Al Lewis, Jude, Bub."
David Baddiel also had an idea for a name, writing on Twitter: "I think, seeing as how he's died since the last one was born, that it would be a fine tribute if they called the new #RoyalBaby Prince. Prince Prince."
Politicians also sent their well wishes following the happy news.
Prime Minister Theresa May sent her "warmest congratulations to the Duke and Duchess of Cambridge on the birth of their baby boy".
"I wish them great happiness for the future," she added.
Mayor of London Sadiq Khan tweeted: "On behalf of Londoners, I want to send our warmest congratulations to the Duke & Duchess of Cambridge on the birth of their baby boy."
Foreign Secretary Boris Johnson offered his "huge congratulations to Their Royal Highnesses The Duke and Duchess of Cambridge on the safe delivery of their baby boy!"
Canada's prime minister Justin Trudeau tweeted: "Canada welcomes a baby boy to the Royal Family!
"Sophie & I send our congratulations to William and Kate, the Duke and Duchess of Cambridge, and to George and Charlotte on the new arrival."