The Duke and Duchess of Cambridge have welcomed a baby prince, following in the footsteps of the Queen and the Duke of Edinburgh.
The new prince arrived at 11.01am (local time) on Monday, weighing 8lb 7oz (3.9kg) Kensington Palace announced.
"Her Royal Highness the Duchess of Cambridge was safely delivered of a son at 1101hrs," Kensington Palace said.
"The baby weighs 8lb 7oz. The Duke of Cambridge was present for the birth.
The self-proclaimed Royalist Town Crier also announced the birth of the baby prince on the steps of the Lindo Wing at London's St Mary's Hospital, marking the moment by saying "God save the Queen", drawing cheers from the crowd of royal watchers.
Emulating the birth pattern of the Queen and Philip's first three children, William and Kate have had a son, a daughter and now another son.
The Queen and Philip had Prince Charles, then Princess Anne, followed by, after a gap of 10 years, Prince Andrew, with Prince Edward born four years later.
Yet William and Kate's third child, a new Prince of Cambridge, arrived after a much closer age gap.
There are only three years between Princess Charlotte and her new baby brother.
Before Charlotte was born in 2015, the Windsors' direct line to the throne had been mostly male-dominated for many years.
The Queen has three sons and one daughter and the Prince of Wales has two sons, William and Prince Harry.
The monarch was one of two sisters but George VI was one of five brothers and a sister.
The arrival of a prince was a surprise for the Duke and Duchess, who chose not to find out the sex of their baby in advance.
Like George, the baby is a Prince of Cambridge.
George was the first Prince of Cambridge to be born for more than 190 years.
The previous Prince of Cambridge, who was also a George, was born in 1819 and was a grandson of George III.
But if the Queen had not stepped in, the new baby would have been Lord (forename) Mountbatten-Windsor, rather than a prince, and would not have been His Royal Highness.
Under past rules, only William and Kate's first-born son was entitled to become a prince after George V limited titles within the royal family in 1917.
The Queen issued a Letters Patent under the Great Seal of the Realm in December 2012 when Kate was around three months pregnant with George, declaring "all the children of the eldest son of the Prince of Wales should have and enjoy the style, title and attribute of royal highness with the titular dignity of Prince or Princess prefixed to their Christian names or with such other titles of honour".
This young prince is also no longer allowed to jump ahead of older sister Charlotte in the line of succession.