Despite the early hour the topic began trending almost immediately on Twitter, with #RoyalBaby and Kate Middleton the top two worldwide.
While many fans gathered at the Lindo Wing others followed and reacted to the news online.
In the last week of the election campaign the party leaders were quick to break away from campaigning by sending their best to the royal couple.
Nicola Sturgeon tweeted: “Many congratulations to Prince William, Catherine and George on the birth of their baby girl. Lovely news.”
While David Cameron wrote: “My best wishes to the Duchess of Cambridge, who is having her second child today. The whole country will wish her well.”
Nick Clegg shared the Prime Minister’s sentiments, saying: “Best of luck to the Duchess of Cambridge today, as she gives birth to her second child. The thoughts of the whole country will be with her.”
Labour leader Ed Miliband said: “I want to pass on my best wishes to the Duchess of Cambridge. My thoughts are with her and the Duke of Cambridge today. I wish them well.”
Author JK Rowling had speculated on Friday that the birth may take place yesterday. She said: “I wonder what odds we’d get on the royal baby being born tomorrow and called Victoria?”
The Lego group took the opportunity to create a new Lego Royal Family, complete the baby princess in a pram, tweeting: “It’s a girl! Congratulations to the Duke and Duchess of Cambridge.”But Twitter user Liz Webb said she was keeping her fingers crossed. “Come on #RoyalBaby. Please be born today and be a princess called Elizabeth.”