The child will be third in line to the throne and is destined to be monarch regardless of its sex, as the British Government is negotiating with all Commonwealth countries to change the law so that a first-born girl can inherit the throne even if she has brothers.

The news was revealed after Kate was admitted to hospital on Monday afternoon for treatment for severe morning sickness. No due date has been announced.

St James's Palace issued a statement announcing the pregnancy and saying, “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.”

Acute morning sickness is normally treated with extra hydration and nutrients, which might include an intravenous drip. Unpleasant though it is for the mother, the received wisdom is that it suggests a healthy pregnancy because it means the level of pregnancy hormones is high. It affects only two per cent of pregnant women.

Only a small number of women are understood to experience the symptoms of Hyperemesis Gravidarum  throughout pregnancy.

‘‘It will mean that the patient may need to be re-admitted throughout their pregnancy... but in terms of any particular complications, if it’s treated well and they’re kept well hydrated it’s something that is relatively easy and well treated,’’ consultant obstetrician Daghni Rajasingham from Britain’s Royal College of Obstetricians and Gynaecologists said

Royal pregnancies are not normally announced until they are passed the fragile 12-week mark and it is thought that the duchess's hospitalisation is the reason for the press release at an early stage. The couple reportedly wanted to avoid speculation about her condition.

News of the pregnancy comes just three days after Catherine visited her former school St Andrew’s, where she ran around a hockey pitch wearing high-heel boots, laughing and playing with children.

It is 18 months since Kate married Prince William and speculation has been rife that a pregnancy might be on the way. A columnist in The Daily Mail last week wrote about the duchess's new haircut and warned that women tend to change their hair when they have a drastic life change, adding, “Predictably… Kate's new cut has sparked speculation that she may be pregnant. (And if she is keeping a Very Important Secret, then that demure fringe is perfect for hiding behind.”)

A remark that now looks as prescient as the not-so-ditsy speculation a couple of years ago about her sudden five-kilo weight loss - which was almost immediately followed by the announcement of the royal engagement.

With hindsight, it seems that an American magazine report last week was the first to break the news. US Life and Style magazine headlined with “A baby is on the way!', citing an unnamed close friend of the couple as the source.

There must have been something in the air. Last week, Prince William accepted a home-made baby suit from a young mother in the crowd as he and Kate toured Cambridge. The suit read, “Daddy's little co-pilot”.

Wellwishers have already signalled joy at the couple’s news, with British Prime Minister David Cameron saying ‘‘the country will be celebrating with them’’.

‘‘I’m delighted by the news that the Duke and Duchess of Cambridge are expecting a baby. They will make wonderful parents,’’ Mr Cameron posted on Twitter, later admitting he learnt of the pregnancy when handed a note during a meeting.

Bookmakers will be taking flurries of bets on what the Duke and Duchess of Cambridge will choose to call the eagerly anticipated new prince or princess.

Speculation as to what they might name their first child began even before Kate was pregnant, with predictions posted ranging from Mary and Matilda to Edmund and George on the mumsnet website before their wedding.

Their decision - be it traditional or unusual - will most likely set a trend for the next generation of babies.

Royal youngsters are mostly given safe, historical names which are passed down through the monarchy such as James, Edward, Charles, George, Mary and Elizabeth.

with agencies