St James's Palace and Clarence House have confirmed the news, following several weeks of rumours of a royal pregnancy.
> Duchess of Cambridge Kate Middleton pregnant: Celebrity reactions
"The Queen, The Duke of Edinburgh, The Prince of Wales, The Duchess of Cornwall and Prince Harry and members of both families are delighted with the news," the royal family said.
"The Duchess was admitted this afternoon to King Edward VII Hospital in Central London with Hyperemesis Gravidarum [acute morning sickness].
"As the pregnancy is in its very early stages, Her Royal Highness is expected to stay in hospital for several days and will require a period of rest thereafter."
"I'm delighted by the news that the Duke and Duchess of Cambridge are expecting a baby," said Prime Minister David Cameron. "They will make wonderful parents."
The Duchess is understood to be less than 12 weeks pregnant, but the news has been confirmed because of her admission to hospital.
Catherine's last public appearance was on Friday (November 30), when she visited St Andrew's School in Pangbourne in Berkshire, where she used to study.
The royal couple's child will be third in line to the throne, and is also in direct succession to the crown.
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