Kate and Prince William are expecting their third child, after Prince George and Princess Charlotte. The baby will be fifth in line to the British throne.
"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 Palace tweeted.
The 36-year-old Duchess travelled by car from Kensington Palace to the Lindo Wing at St. Mary's Hospital with The Duke of Cambridge, it said.
The mother-of-two is likely to give birth to her third child on St Georges Day, British media reported.