Duke and Duchess of Cambridge expecting their third child
In a statement the palace said the duchess was suffering from severe morning sickness, as she did with her previous pregnancies, so a public engagement she had later on Monday had been cancelled.
The statement said: "Their royal highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their third child.
"The Queen and members of both families are delighted with the news.
"As with her previous two pregnancies, the Duchess is suffering from Hyperemesis Gravidarum.
"Her royal highness will no longer carry out her planned engagement at the Hornsey Road Children's Centre in London today.
"The Duchess is being cared for at Kensington Palace."