The new royal baby boy has created history and will slot in as fifth-in-line to the throne.
Prince William and his wife Kate's third child means the new line of succession will see the new Prince of Cambridge follow the Prince of Wales, Duke of Cambridge, Prince George and Princess Charlotte.
Historically, Princess Charlotte would have lost her spot to her younger brother, but legislation introduced in 2013 overhauled the order of succession.
The newborn has also pushed Prince Andrew down to seventh-in-line, meaning he will not have to ask his mother's permission if he wanted to re-marry.
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According to the new legislation, the first six people in the line of succession must obtain the Queen's permission to marry.
Previously, under a law dating back to 1772, all descendants of George II had to seek the monarch's permission.
Kensington Palace announced the news on Twitter and said: "Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 11.01am.
"The baby weighs 8lbs 7oz. The Duke of Cambridge was present for the birth. Her Royal Highness and her child are both doing well.
"The Queen, The Duke of Edinburgh, The Prince of Wales, The Duchess of Cornwall, Prince Harry and members of both families have been informed and are delighted with the news."
The Queen, who turned 92 this weekend, is Britain's longest reigning monarch and has ruled for 65 years.
As the child of a royal "spare", the Queen was thought unlikely to reign until her uncle, King Edward VIII, abdicated in favour of her father in 1936, whereupon he became King George VI and she the heir to the throne.