The Duke and Duchess of Cambridge are expecting their first baby, it has been announced.

St James's Palace revealed the news after the duchess was admitted to hospital on Monday afternoon with acute morning sickness.

It is understood that the duchess is not yet 12 weeks pregnant. But the fact that she has been admitted to hospital, and is likely to stay there for several days, forced the palace into making an announcement earlier than it would have liked.

In a statement, a spokesman said: "Their royal highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting a baby. 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 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."

Hyperemesis gravidarum is acute morning sickness, which requires supplementary hydration and nutrients.

The baby will be the third in line to the throne, bumping Prince Harry, the baby's uncle, to fourth place.