The Queen and both families are said to be "delighted with the news".
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.
This is published unedited from the PTI feed.
- Britain's Royal Family is preparing to welcome a new baby.
British royals Prince William and Catherine, the Duke and Duchess of Cambridge, have announced that they are expecting their third child. Prince George is set to begin his first day of school on Thursday, September 7, at Thomas's Battersea in south London.More news: Mahrez becomes late transfer target for Mourinho
It was not immediately clear when the baby is due although betting agencies were beginning to offer odds on possible names for the soon-to-be-born child in the House of Windsor. She is being cared for at their London home.
And Kate also gave an extremely telling clue that she wants more children in the future in Poland.
Kate is the eldest of three siblings, and reportedly had a very happy childhood.
Saying thank you for the present, she turned to the Duke, laughing, and said: 'We will just have to have more babies'. William has a younger brother, Prince Harry.