The Duke of Cambridge, who was by Kate Middleton's side as she gave birth, also phoned his brother Prince Harry and his father Prince Charles.
"He also made sure that the Middleton family were fully aware," said Sky's royal correspondent Paul Harrison. The Queen was at Buckingham Palace when she received the news.
Both families had been kept informed of Kate’s progress. They also kept the news to themselves, and the press release about the royal birth was sent out around four hours after the baby was born.
William and Kate's son was born at 4.24pm and the news was not revealed on a press release until 8.30pm. The news was officially announced on an easel outside Buckingham Palace around 20 minutes later.
"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," said a statement from Kensington Palace.
"Her Royal Highness and her child are both doing well and will remain in hospital overnight."
The baby, who will be a Prince of Cambridge, is the Queen’s third great-grandchild. Prince William’s cousin Peter Phillips has daughters Savannah, aged two, and Isla, aged one.