The Duchess of Cambridge has given birth to a boy, her third child and second son.
The new arrival, who was born at St Mary’s Hospital, London, is fifth in line to the throne and the Queen’s sixth great-grandchild.
The Duke of Cambridge was present for the birth of his son at 11:01 BST, weighing 8lb 7oz.
A Kensington Palace statement said Catherine and her son, who is yet to be named, are “both doing well”.
Members of both families had been informed and were “delighted with the news”.
Kensington Palace said the name of the baby would be announced in due course while a formal notice of the birth had been posted on the forecourt of Buckingham Palace.
Favourite names at the bookmakers include Arthur, Albert, Frederick, James and Philip.
Prime Minister Theresa May tweeted her “warmest congratulations” to William and Catherine.
She said: “I wish them great happiness for the future.”
The Archbishop of Canterbury added: “Congratulations to The Duke and Duchess of Cambridge on the safe delivery of their baby, a brother for Prince George and Princess Charlotte.
“May God bless them and all of their children with love, happiness and health.”
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.”
Sir Vince Cable, leader of the Liberal Democrats, said: “Congratulations to the Duke and Duchess of Cambridge on the birth of their son.
“I’m sure his older brother will be delighted he has been born on St George’s Day!”
Following a change in the law in 2013, Princess Charlotte keeps her place as fourth in line to the throne.
Previously, younger royal males would have taken precedence over older female siblings.
The duchess was admitted to hospital shortly before 06:00 on Monday with the announcement of the birth coming shortly after 13:00.