LONDON - Britain's Prince William and his wife Catherine are expecting a baby, destined to be the country's future monarch, although the mother-to-be is in hospital with a type of very acute morning sickness that sometimes indicates twins.

"Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting a baby," the prince's office said in a statement on Monday, adding that Queen Elizabeth and the royal family were delighted.

Britain's Prince William leaves the King Edward VII hospital in London, Dec 3, 2012. Britain's Prince William and his wife Catherine, Duchess of Cambridge are expecting a baby, the prince's office said on Dec 3, 2012. The statement said Catherine was in the King Edward VII Hospital in Central London suffering from very acute morning sickness. [Photo/Agencies]

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

"It's only been a matter of time. Everyone has been waiting for Kate to announce that she was pregnant," Claudia Joseph, who has written a biography of the duchess said.

A spokeswoman for the couple said 30-year-old Catherine, widely known as Kate, was in the King Edward VII Hospital in central London suffering from Hyperemesis Gravidarum, an acute morning sickness which causes severe nausea and vomiting and requires supplementary hydration and nutrients.

Professor Tim Draycott, a consultant obstetrician at the University of Bristol, said the condition was common in the early weeks of pregnancy but did not put the baby at any increased risk, although in extreme cases it can lead to the baby being born with a slightly low birth weight.

Draycott said it may also indicate more than one royal baby may be in the offing.

"Hyperemesis is slightly more common with twins," said Draycott, explaining that the condition affected around one in 100 to 200 pregnant women.

William, a Royal Air Force helicopter pilot, was at her side and she is likely to remain in hospital for several days. There was no detail about when the baby was due, although the prince's spokesman said she was less than 12 weeks pregnant.

"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."

BABY WILL BE KING, OR QUEEN

William, Queen Elizabeth's 30-year-old 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 males 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.

Britain's royal family are currently riding the crest of popularity on the back of William and Kate's wedding and the queen's diamond jubilee this year which has witnessed nationwide celebrations.

"It's something everyone can look forward to, just like their wedding brought the whole nation together," said Johanna Castle, 25, a sales assistant in an east London homewear and fashion store.