London's BT tower delivered the news on the arrival of a new royal baby on Saturday (2 May).
Flashing at the top of one of the city's tallest landmarks, the message read "It's a girl" and could be seen from all over central London.
Britain's Duchess of Cambridge, the wife of Prince William, gave birth to a girl on Saturday, the couple's second child and a sister to one-year-old Prince George.
The royal family's newest member was born at 8.34 am, some 2-1/2 hours after Kate Middleton was admitted in the early stages of labour to St Mary's Hospital, West London, the couple's Kensington Palace residence said in a statement.
The baby princess will be fourth in line to the throne behind her brother George, father William and grandfather Prince Charles, pushing her uncle Prince Harry down a place in the royal hierarchy.
The baby, whose name has yet to be announced, weighed 8 lbs 3 oz and William was present at the birth, the palace said. "Her Royal Highness and her child are both doing well," it added.
Both families of the couple had been informed, including the new baby's great-grandparents Queen Elizabeth and Prince Philip. She becomes the 89-year-old queen's fifth great-grandchild.