Prince William and Kate expecting a baby

"Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting a baby," the office said in a statement.

"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 couple, officially known as the Duke and Duchess of Cambridge, married in April last year, prompting a global media frenzy and there has been much speculation, particularly in U.S. gossip magazines, about a possible pregnancy.

The statement said Catherine was in the King Edward VII Hospital in Central London suffering from very acute morning sickness. She is likely to remain there for several days.

It said her condition required supplementary hydration and nutrients. There was no detail about when the baby was due, with the statement stating the pregnancy was in its early stages.

"I'm delighted by the news that the Duke and Duchess of Cambridge are expecting a baby," Prime Minister David Cameron said on his Twitter website. "They will make wonderful parents."

The Duchess of Cambridge didn't let her dress or heels keep her from showing off her athletic prowess on a new hockey field at her alma mater last week.

The Duchess of Cambridge didn't let her dress or heels keep her from showing off her athletic prowess on a new hockey field at her alma mater last week.

William, Queen Elizabeth's grandson, is second in line to the British throne, and their first child will become the third in succession when he or she is born.

Last year Britain and other Commonwealth countries which have the queen as their monarch agreed to change the rules of royal succession so that sons would no longer have precedence as heir, regardless of age.

The agreement also means an end to a ban on a future monarch marrying a Catholic, a stipulation dating back some 300 years.

William and Catherine currently live in north Wales where the prince is based as a Royal Air Force search and rescue pilot.