Prince William's wife Catherine is pregnant with their first child, St James's Palace said on Monday, in an eagerly awaited announcement about a baby destined to be Britain's future king or queen.
The former Kate Middleton is in hospital suffering from severe morning sickness, the palace added in a statement which ended months of fevered speculation about whether she was expecting a baby.
Queen Elizabeth II and the rest of the royal family are "delighted" by the news, as are the Middletons, it said.
"Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting a baby," St James's Palace said in a statement, using the couple's official titles.
William, 30, the son of heir to the throne Prince Charles and the late Princess Diana, married Catherine, also 30, in April 2011 in a ceremony watched by billions of people around the world.
The palace refused to say exactly when the couple became aware she was expecting a baby, saying that it was only "recently".
But Britain's Press Association news agency reported that the pregnancy has not passed the 12-week point, the traditional time for making a pregnancy public, and the announcement was prompted by Kate's medical condition.
"The Duchess was admitted this afternoon to King Edward VII Hospital in Central London with Hyperemesis Gravidarum," the statement said.
It defined the condition as "very acute morning sickness, which requires supplementary hydration and nutrients".
"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.
The statement said the queen, her husband Prince Philip, William's father Prince Charles, Charles's wife Camilla, William's brother Prince Harry "and members of both families are delighted with the news."
The baby would be third in line to the throne after Charles and William, irrespective of whether it is a girl or a boy following a change of the rules of succession last year.
Prime Minister David Cameron said he was "delighted" and the royal couple would make "wonderful parents".
Speculation about a royal pregnancy intensified last week when the couple were given a baby's romper suit during their first visit together to Cambridge, the historic English university city which is home to their dukedom.
William laughed and said "I'll keep that" after accepting the tiny hand-stitched outfit with the words "Daddy's little co-pilot", a reference to his job as a Royal Air Force search and rescue helicopter pilot.
The baby news comes just over one year since Commonwealth nations agreed to scrap centuries-old laws barring first-born daughters from inheriting the British throne.
The 1701 Act of Settlement had formalised the policy of male primogeniture, although the tradition dates back even further to feudal times and beyond.
Under the law, female heirs to the throne had been passed over if younger male heirs were available.
The queen -- whose husband Prince Philip is officially titled the Duke of Edinburgh -- was crowned in 1953 only because she did not have any brothers.
Royal commentator Kate Williams said William and Kate's child would be unique due to its place as the first possible heir to be born after the change in the succession laws.
"I think what's politically and constitutionally important is that this child, whether it's a little girl or a little boy, will come to the throne," she told the BBC.
Williams added it was an "ideal time" for the couple to have a baby following months of public duties with the London Olympics and the Diamond Jubilee marking the queen's 60th year on the throne.
The speculation about a baby has been particularly intense because Britain's royals have had a habit of producing heirs to the throne soon after marrying.
William was born barely 11 months after his mother Diana married his father Prince Charles in 1981. Diana was killed in a car crash in Paris in 1997 and Charles later married Camilla, now known as the Duchess of Cornwall.
And Charles himself was born almost exactly a year after his parents the queen and Prince Philip married in 1947.
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