The royal revelation came as Catherine, Duchess of Cambridge, was admitted to a central London hospital with acute morning sickness Monday afternoon.
She is expected to be in King Edward VII Hospital for several days, the palace announcement said.
William and Catherine were married on April 29, 2011, in London’s biggest royal wedding in three decades.
The baby would be the first grandchild for the heir to the British throne, Prince Charles, and the late Diana, Princess of Wales.
“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,” the statement said.
She is not yet 12 weeks pregnant, Clarence House told CNN, so the palace is not announcing a due date for the child. Prince William is at the King Edward VII hospital with Catherine, the palace says.
The news immediately became a top trending topic worldwide on Twitter.
British Prime Minister David Cameron was quick to tweet his joy: “‘I’m delighted by the news that the Duke & Duchess of Cambridge are expecting a baby. They will make wonderful parents.’#royalbaby”
British Labour Party leader Ed Miliband tweeted “Fantastic news for Kate, William and the country. A royal baby is something the whole nation will celebrate.”