The Queen and both families are said to be "delighted with the news."
The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their third child pic.twitter.com/DZCheAj1RM
As with her previous two pregnancies, the Duchess is suffering from hyperemesis gravidarum, or severe morning sickness.
She will no longer carry out her planned engagement at the Hornsey Road Children's Centre in London today.
Catherine is being cared for at Kensington Palace, the statement said.