THE Duke and Duchess of Cambridge are expecting a baby, St James's Palace announced today.
The statement also said that William's father Prince Charles and his grandmother Queen Elizabeth II are thrilled with the news.
"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," it continued.
But St James's Palace added 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."
Hyperemesis gravidarum is a rare condition which causes severe vomiting during pregnancy.
The severity of the vomiting can cause dehydration, weight loss and a build-up of toxins in the blood or urine called ketosis.
It affects 3.5 per 1,000 pregnant women and can cause women to vomit blood.
Symptoms also include severe nausea, low blood pressure and fast heart rate, headaches, lethargy or confusion.
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 today's announcement was prompted by the Duchess's medical condition.
Reacting on Twitter, British Prime Minister David Cameron wrote: "I'm delighted by the news that the Duke and Duchess of Cambridge are expecting a baby. They will make wonderful parents."