Britain's future Queen, Kate Middleton, is pregnant with her second child.
As with her first pregnancy, the duchess is suffering from very acute morning sickness and is being treated by doctors at Kensington Palace.
Prince William's wife had given birth to baby George last July.
A Clarence House statement on behalf of the Duke and Duchess of Cambridge said Queen Elizabeth II and members of both families were delighted with the news.
Prime Minister David Cameron tweeted he was "delighted by the happy news that they're expecting another baby".
Kate, 32, will no longer accompany Prince William on their planned engagement in Oxford today, Clarence House said.
The sickness, called hyperemesis gravidarum, is a condition that may require supplementary hydration, medication and nutrients.
Clarence House said: "Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their second child.
