According to the official Twitter account for the Prince of Wales's office, Clarence House:
Her Royal Highness the Duchess of Cambridge was safely delivered of a son at 4:24 PM today. The baby weighs 8 lbs, 6 oz. The Duke of Cambridge was present for the birth. The Queen, the Duke of Edinburgh, the Prince of Wales, the Duchess of Cornwall, Prince Harry, and families have been told and are delighted. Her Royal Highness and her child are both doing well and will remain in hospital overnight.
Congratulations to the Duke and Duchess of Cambridge!