Kate has given birth to a boy weighing 8lbs 7oz.
The 36-year-old royal and her husband Prince William – who already have son Prince George, four, and two-year-old daughter Princess Charlotte – welcomed their third child into the world at 11.01am on Monday.
Kensington Palace announced: “Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 1101hrs.
“The baby weighs 8lbs 7oz (about 3.8kg).
“The Duke of Cambridge was present for the birth.
“Her Royal Highness and her child are both doing well.”
Both William and Catherine’s families are “delighted” with the new addition to the household.
The statement concluded: “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.”
It was revealed earlier in the day that Catherine – who was known as Kate Middleton before her marriage – had been admitted to hospital after going into labour.
Kensington Palace announced: “Her Royal Highness The Duchess of Cambridge was admitted to St. Mary’s Hospital, Paddington, London earlier this morning in the early stages of labour.
“The Duchess travelled by car from Kensington Palace to the Lindo Wing at St. Mary’s Hospital with The Duke of Cambridge.”
Catherine carried out her last official royal engagement last month at the Team SportsAid event in London and will be on maternity leave until the autumn.
An insider said recently: “Kate is now officially on maternity leave until the autumn and looking forward to welcoming the new baby.
“The whole family is excited, including the children, who are very much aware that they are about to welcome a new brother or sister.”