PRINCE William and his wife, Catherine, have announced they are expecting a baby.

It ends days of speculation the 30-year-old duchess was pregnant.

The announcement came after Kate, 30, was taken late yesterday afternoon to King Edward VII Hospital in London where she is expected to remain for several days with severe morning sickness.

But the news was met with great joy from the royal households and the general public.

The couple put out a statement saying they were "delighted with the news".

"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,” their statement said.

The Daily Mail reports that that members of both the Royal Family - including the Queen - were only told about the pregnancy after the decision was made to admit Kate to hospital.

The Middletons were, however, already aware given her condition this weekend.

The baby will be born third in line to the throne, thanks to laws changed last year in Britain, and it will be the first time in almost 120 years a serving monarch gets to meet a great grandchild born directly into succession to the crown.

When the newest addition to the Royal Family arrives, Prince Harry, the baby's uncle, will be bumped down the line of succession to fourth place.

Prime Minister David Cameron was the first to congratulate the couple taking to Twitter to state "they will make wonderful parents”.

After the announcement St James's Palace issues a statement saying Kate had hyperemesis gravidarum, a rare condition that causes severe vomiting, often blood, and affects 3.5 in every 1000 pregnant women.

Symptoms also include severe nausea, low blood pressure and fast heart rate, headaches, lethargy or confusion.

"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,” St James’ Palace announced.

Rumours of a pregnancy had been rife for weeks but hit a crescendo last week with many taking to the internet to discuss the future royal baby.

Then Prince William on a tour of Cambridge was handed a monogrammed baby jump suit which added to the story.

When News Ltd contacted the palace last week, the rumour was dismissed as just speculation. It is thought the announcement of her pregnancy had to be made weeks early after she took ill.

Just last Friday Kate attended her old school and was photographed in a picture of health showing off her hockey skills and chatting to students.

Royal births are usually celebrated with a Royal Salute of 41 guns.

They are registered in the normal way, although the Home Secretary is required to notify certain officials including the Lord Mayor of London, while the Queen's Private Secretary Sir Christopher Geidt informs Governor Generals overseas.

There is also the age old custom of attaching Notice of Royal births and deaths to the railings at Buckingham Palace for members of the public to read.

But as well as keeping to this tradition, the arrival of royal birth will no doubt also be officially declared via the monarchy's websites, on Facebook and via Twitter.

Joe Little, managing editor of Majesty magazine, said the news bookended a year that saw the royal family riding high in popular esteem after celebrations of Queen Elizabeth II's 60 years on the throne.

"We're riding on a royal high at the moment at the end of the Diamond Jubilee year," he said. "People enjoyed the royal romance last year and now there's this. It's just a good news story amid all the doom and gloom."

Speculation about when the couple would start a family has been rife since their wedding.

William's mother - the late Princess Diana - got pregnant just four months after her wedding in 1981. Diana reportedly suffered from morning sickness for months and complained of constant media attention.

"The whole world is watching my stomach," Diana once said.

Read more on the Herald Sun.