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 added in a statement that Kate had been admitted to King Edward VII Hospital in central London for treatment for a severe form of morning sickness, known as hyperemesis gravidarum.
Kate was taken to the hospital by car from the home of her parents in Bucklebury in Berkshire. She is expected to remain there for a few days and her engagements for the rest of the week have been cancelled.
– St James's Palace spokesman
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 confirmed William had been by his wife's bedside throughout the day and was later seen leaving the central London hospital:
Following news of the announcement, politicians and celebrities took to Twitter to wish the royal couple their best wishes for the pregnancy:
I’m delighted by the news that the Duke and Duchess of Cambridge are expecting a baby. They will make wonderful parents.
I'm sooooooo excited that we're having a royal baby !!!! Congratulations to Kate and Wills !!!!!!
Fantastic news for Kate, William and the country. A royal baby is something the whole nation will celebrate.
Congrats Wills and Kate....Great news.The calpol is in the post.
Last week bookmakers suspended betting on the Duke and Duchess of Cambridge having a baby in 2013.
David Stevens, of Coral, said: "We saw numerous bets for a Royal new arrival throughout the Jubilee summer, and the betting went through the roof again when Prince William accepted a babygro while on a tour of Cambridge."
During the same week Kate was seen playing hockey at her old prep school, St Andrew's in Pangbourne, Berkshire. A spokesperson for the school said they had "no inkling" the Duchess was pregnant, adding: "Obviously everyone at St Andrew's are delighted for them both. We're absolutely thrilled."
The Duchess of Cambridge reportedly found out she was pregnant last month and wanted to wait until the 12 week point until making an announcement, according to The Daily Telegraph.
A spokesman for the Royal Air Force (RAF), with which William serves as a search-and-rescue pilot, has also added their congratulations to the couple:
The RAF is delighted with the news and wishes the Duke and Duchess of Cambridge all the best for the future.
Prince William's uncle Earl Spencer has expressed his delight following the announcement of the Kate's pregnancy.
The brother of Princess Diana said in a statement that "it is wonderful news and I am thrilled for them both."