A future monarch: William and Kate expecting a child

The Duke and Duchess of Cambridge are expecting a baby, St James's Palace announced today.

The couple are currently in hospital, where Kate Middleton is suffering a severe form of morning sickness.

"It's absolutely wonderful news. I'm delighted for them," prime minister David Cameron said.

"I'm sure they'll make absolutely brilliant parents." 

Labour leader Ed Miliband commented: "The whole country will be thrilled. Justine and I wish Kate and William our warmest congratulations."

It is unclear how long the princess has been pregnant. However, it appears she only became aware of it recently and that she has not passed the 12-week point.

The couple were "very pleased", St James's Palace 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," they said.

"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 hydration and nutrients.

The news came as a shock to royal observers, although rumours about a possible royal baby have been circulating since the couple married nearly two years ago.

The baby will be the first to be made king or queen regardless of their sex. Under changes to the rules of primogeniture being enacted by the coalition, an older female baby would be next in line to the throne regardless of how many younger brothers she has.

The reforms have not been passed yet, but they will be backdated to October 2011.

When the baby is born, it will automatically become third in line to the throne, after Prince Charles and Prince William. Prince Harry will become fourth in line to the throne.

Barring unexpected events, it will become head of the armed forces, supreme governor of the Church of England, head of the Commonwealth and head of state of 16 countries.

The royal family may have released the news because of the inevitable media attention arising from the Duchess of Cambridge's attendance in hospital.

But the shock the news produced showed how well the royal family had managed to keep control of the news agenda and prevent leaks.

The baby's birth will be the first time a still-serving monarch will meet their great grandchild born in direct succession to the crown for nearly 120 years.