The Duke and Duchess of Cambridge are expecting a baby, St James's Palace has announced.
The Queen and Duke of Edinburgh, the Prince of Wales and members of both families are delighted with the news, said a spokesman.
But St James's Palace said in a statement that Kate had been admitted to a private hospital for treatment for a severe form of morning sickness known as hyperemesis gravidarum.
St James's Palace said in the statement: "The Duchess was admitted this afternoon to King Edward VII Hospital in central London with hyperemesis gravidarum.
"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."
St James's Palace would not say when the royal couple became aware of the pregnancy only saying "recently".
It is understood that the pregnancy has not passed the 12-week point and the announcement was prompted by the Duchess's medical condition.
For women with hyperemesis gravidarum their vomiting can be so severe they cannot keep food or liquid down.